What are the power-related limitations of a USB-powered 2.5" hard drive enclosure? Some external hard drives require an extra power source, while my current one does not require one.
Is there a certain limitation (Disk speed/RPM, storage size (500 GB>), specific power consumption) for enclosures where the power source is via the USB?
Generally, no
Specifically, some enclosures can have limits relating to the SATA/IDE controller built into the enclosure.
All 2.5 inch HDDs I've seen have roughly the same power consumption (depending on 5200 or 7200 RPM mainly). Every single one I could operate with the 2 USB cables included with the external hard drive cases. If I plug it into the back of the PC I often only needed one USB cable. Using plugs on USB hubs will often cause problems, though.
USB sockets, per specification, can output up to 500 mA.
Practically 2.5" hdds could use from 600 to 1100 mA for running up. The high power consumption for this short time is - most of the time - allowed by motherboards. When running they will need about 250 to 400 mA, so they are way into the USB specification there.
3,5" hdds will NEVER work from USB alone, since they need +12V power and for a conversion there just aren't enough ampers!
A few third party 2.5" enclosures I've bought have included "Y" cables with an extra USB connector to help it draw additional power; a Google Images search shows lots of examples.
Even with these enclosures however, from my own experience 2.5" hard drives can get by with the rated power in a single typical USB port, even in the mean old days when USB 1.0 was exciting and 8.4GiB drives seemed luxuriously accommodating. If the enclosure uses Parallel ATA, it can also accept 1.8" adaptors and Microdrives without power issues.
This isn't the case with 3.5" drives, which weren't designed with portable computers and batteries in mind, but rather for desktops with larger and more consistent power supplies.
If the drive is a 2.5" drive, there should be no limitation. After all, virtually all of them can plug into the same laptop using the same amount of power, regardless of size or speed, no?
I will say this: USB is +5V, and if your USB port is not well-engineered (or not working properly) to put out this expect power, you could have problems as some here have had in the past. We have seen USB ports in questions putting out less than +5V, causing problems for the user. The solution in that case is often a powered USB hub.
Check out this previous post: How do you check how much power a USB port can deliver?
