I’m reading through the manual for a Samsung portable USB hard drive, and I’ve come across this warning:
Detaching the USB cable while file transfer is in progress can damage your computer and/or [hard drive].
Here’s the original warning:
Seriously? I’m going to fry my computer by doing something USB was explicitly designed for right from the beginning—hot-plugging? Surely a computer doesn’t become damaged just because a data transfer fails.
That claim of damage is more of a legal way for a manufacturer to protect themselves in the off-hand chance something goes wrong. Meaning it is practically nonsense. It’s no different than the obtuse language used in an EULA (End-User License Agreement) in the world of software; the manufacturer is right, you are wrong, our company has better lawyers than you, give us your money for a product, have a nice day, goodbye.
I highly doubt anything would be seriously damaged by removing a USB hard drive that is still active. So I wouldn’t live my life in fear of a warning like this.
I would however take this nonsense to mean if something did go wrong the manufacturer would avoid any responsibility or liability instead claiming the fault is yours since you failed to properly use the device as instructed in that little pamphlet most people immediately throw out when they purchase such a device.
More details below.
Seriously? I’m going to fry my computer by doing something USB was explicitly designed for right from the beginning—hot-plugging? Surely a computer doesn’t become damaged just because a data transfer fails.
Will you fry your whole computer? Most likely no. Do you increase the risk—even on a slight level—of damaging a USB port by unplugging the drive while it’s active? I’d say yes. The risk mainly comes from something like static electricity being generated between you, the computer, the USB cable and the drive port. And since the drive is powered and connected to some level of grounding, it becomes an attractive path for a stray static charge as explained in this blog post on the “Premium USB Blog”; bold emphasis is mine:
ESD damage to your USB drives or ports can cause latency failures that will slow down your data transmission following static shock. Your port or device could also face more severe damage that would essentially fry it and cause it not to work at all. It could also shorten its overall lifespan. ESD can occur under simple circumstances—plugging and unplugging or flipping a nearby switch.
Before you alarmingly back away from your computer, be comforted by the fact that the average computer user won’t necessarily have to worry about it happening. However, it’s still important to know about it in case you ever find yourself in a situation where ESD may occur. The odds of static shock increase once you add computer system upgrades, USB hard drives, graphics cards and other heavy duty peripherals.
Because USB ports are hot pluggable, they need protection against static shock. With USB 2.0 data transfer rates up to 480Mbps and USB 3.0 at 5Gbps, these speeds are fast enough to spark interruptions in the signals. Many USB hubs are protected from ESD up to 2kV, but that’s not always enough.
But again the risk is slight but—as that article explains—the risk does exist on some level.
Another risk is gyroscopic inertia that stems from the hard drive itself still spinning while you disconnect it. I’ve actually lost two external 3.5" drives because I unmounted them, disconnected them and grabbed them too quickly while the drives themselves were still spinning. The disorientation from me lifting one way but the gyroscopic inertia from spinning platters pulling in another direction resulted in me losing my grip on the enclosure with the drive coming crashing down onto the floor and basically making the drive unusable. But again, this is an edge risk.
If you ask me, overwrought details like this in the manuals are not warning you of a common risk as much as they are limiting the drive manufacturer’s liability if somehow your data is lost—or the drive dies—and you end up complaining to the manufacturer. The idea is you would conceivably call up a support tech, they would ask you what you did, you might tell them you disconnected the drive in the middle of a transfer and then they might say, “Sorry, but we don’t cover that.”
Remember, much like an EULA (End-User License Agreement) the main purpose of such documentation is not to make sure you read it as much as it is in the manufacturer’s best interest to make sure they can claim you were provided these “warnings” if something goes wrong from a legal standpoint.
Also, just a clarification on this point:
I’m going to fry my computer by doing something USB was explicitly designed for right from the beginning—hot-plugging?
The idea that USB devices can be hot plugged into the system simply means the USB connection is “hot pluggable” but how the devices past that basic connection reacts if hot plugged is a different issue entirely.
For example, I just updated my iPhone while writing this post and I had it connected to my Mac Mini via a USB cable. Sure, I could have unplugged the USB cable in them middle of the update… And what would happen? My computer would be fine and my iPhone would technically be fine from a physical standpoint. But if I unplugged it at the perfectly wrong moment, I could have “bricked” my iPhone.
A device being “hot pluggable” simply means that the device can be connected without having to completely power down the main system to connect it or use a device “probe” like those SCSI probe control panels most every Mac OS install had back in the pre-Mac OS X days of the OS.
I believe the technical term is "covering your rear".
Its unlikely you will damage your pc, and somewhat unlikely you'll kill your hard drive (though sudden power stops are bad).
However, you might cause data loss, if a transfer is in progress.
That said, if the drive does die, this gives them grounds to deny a warranty request if you mention doing that, and pretty much covers them if there's data loss. "It says so in the manual, you were using it wrong"
Ejecting properly is best practice but hardly a matter of life and death.
If you are talking about an actual hard drive with a spinning motor (eg. an inductive load), you shouldn't discount the possibility of an inductive voltage spike when disconnecting power:
https://electronics.stackexchange.com/a/19868
Obviously both the drive AND the computer should be designed to withstand it, but it is definitely possible that the hard drive motor, powered via USB, could generate enough of an inductive "kick-back" when disconnected to damage the drive AND/OR the computer.
So naturally, as other users mentioned, they're covering their liability.
Edit:
Here is what an inductive voltage spike looks like on a much larger scale:
They might be afraid that you could copy system files from your USB drive onto your main hard drive (from a backup, say): detaching your USB drive in the middle of a transfer could leave you with an incomplete copy of a system file on your hard drive that would then no longer work, thus "damaging" your computer. Think about an incomplete explorer.exe on Windows or bash on Linux, and the effect they would have on system startup.
"while file transfer is in progress" which means that the file structure would be in flux and could get corrupted if it doesn't have some kind of transaction system. Using "Safely Remove Hardware" will flush any delayed writes, wait until the transfer is complete and ensure no programs are using files on it.
I have personally seen a usb drive fail to mount on unix after being pulled out of a Windows machine without "Safely Removing" it. Putting it back into the widows machine and doing a safe remove fixed it.
My understanding is that the USB standard says that if a device is active, then electricity may be flowing to and from that device, and the device is allowed to count on the idea that the electricity may continue. This is a key reason why operating systems have support to "safely" turn off a USB device; the OS tells the USB device to start expecting that the electricity may become unavailable.
Any time you have electricity being transferred, you shouldn't just separate the electrical connection. In theory, the electricity could jump a small amount of air, and might find a different electrical contact.
In practice, the risk seems small for USB-based thumb drives, because people do that all the time. In practice, this DOES occasionally damage the thumb drive. When I was actively working tech support for dozens of companies (simultaneously; I worked for a company that other companies used for tech support), I don't ever recall hearing of the computer being damaged. So it must be exceedingly rare, but I think that the potential does theoretically exist.
This is still called "hot pluggable", because the computer remains hot: you're not required to shut down the entire computer.
Some USB implementations have done an extremely poor job of handling unexpected device disconnection. Samsung has no way of knowing whether its drive might be used with such an implementation. Consider the following sequence of events:
Application #1 is talking to drive. Code between the application and kernel layers asks for a buffer where it should put data to be sent to the drive and starts putting data into that buffer.
The OS scheduler decides to give some CPU time to some other application while #1 was still writing to that buffer.
Someone unplugs the USB drive.
The system notices that the drive is no longer plugged in and releases the buffers that were allocated to it.
Something else requests a buffer for some entirely separate purpose and receives the buffer formerly used by application #1.
Application #1 gets another time slice and continues writing data to what used to be its buffer.
Even on a really sloppy USB implementation such a sequence of events would be unlikely to occur, since everything would have to happen at certain specific times, but there's no telling what could happen if events unfolded as described. Since it's possible that the USB implementation might work perfectly and reliably if nothing is ever unmounted unexpectedly, but an unexpected dismount could cause unbounded disaster, improperly dismounting of a device should be considered a possible cause of disastrous failure on some systems. Since Samsung has no idea whether users of its drive might be plugging them into such systems, it errs on the side of caution by avoiding any assumption that they aren't.
Computer probably not, but the drive could be damaged.
The spinning hard drive normally moves the heads to the parking position before stopping. On unexpected powering off, this does not happen, and the heads drop on the disk plates where the data are stored. This may damage heads, disk plates or be a reason of data loss. As the heads fly above the plates also while reading, unplugging a reading hard drive is also very bad.
While writing, also the file allocation table may go out of sync with content if not updated.
The SSD hard drive takes complex actions while writing: it may need to erase relatively large memory blocks, move data to consolidate them into one block, maintain internal indexes and tables while doing this. Apart the file allocation table that it also has, a SSD drive maintains a complex "internal" housekeeping.
Unexpected power loss may leave it in inconsistent state. Enterprise grade SSDs have super capacitors that have enough energy to support a clean shutdown, but USB sticks and mainstream SSDs usually do not.
Hence SSD and USB stick damage is more likely when the device is doing intensive writing. It is much less probable if the device is reading or inactive.
I am not aware of any reasons why the computer should be damaged.
Under ideal conditions, no. However, a spinning mechanical hard drive contains a lot of energy, and a three phase motor that acts as a generator during spin-down. The drive should actively manage this process, but you've just pulled the data lines without shutting the drive down first, so it is probably using diodes to shunt the power back into the motor - essentially shorting the motor windings, which will slow it down faster and dump the heat into the diodes.
Note that the power lines are still connected for several dozen milliseconds after the data lines are disconnected.
In an imperfect world, diodes fail, and a high speed set of platters contain a substantial amount of energy. Under the wrong circumstances it's unlikely, but possible, that this energy will make it to the USB power pins. It will have to pass through some fairly sophisticated defenses to prevent that from happening, but in a cheap drive with a cheap USB/SATA chipset, it's possible to feedback too much energy into the USB power supply.
And, in most computers, the USB power supply isn't as well protected as the USB specification suggests it ought to be. Still, there are protection diodes here that shunt power back to the power supply where it should be able to handle over and undervoltages. Cheap power supplies don't like to do that, though, and a few dozen milliseconds of high voltage on the USB power pins before they too are disconnected could be very bad for the rest of the computer's components sharing the 5V line.
Fortunately the computer makes most use of the 3.3v and 12v lines, so damage may be limited, and further may not completely cause the computer to fail, but simply to become flaky.
So while it's very, very, very unlikely that this would happen, it's still within the realm of possibility - and in addition to data loss, you could experience some hardware damage.
You can add additional protection, and completely isolate the USB device galvanically, but there are a multitude of protections already in place and I wouldn't fret over it. However, I'd still strongly suggest that you use the operating system's disconnection operation for mechanical drives, rather than relying on the hot plug capability.
It most probably won't but it indeed can. Any electromagnetic event has a chance, unexpected ones especially. This has actually happened to me once about 15 years ago - my laptop mainboard has got semi-fried after using a USB hard drive (a seemingly healthy one, I still use it occasionally and it's ok, no incidents have happened to it or been caused by it since then), USB ports became USB 1.1-only and memory quirks (observable with memtest86 and causing occasional unexpected behaviour during the computer usage) began to happen. I've had to apply warranty to replace the mainboard.
