What does the 2.5"/3.5" value indicate? The thickness?
I want one of my friends to buy 1TB hard disk for me. I know I want a 1TB capacity and 7200rpm, but don't know how to choose between 2.5" and 3.5"; I am worried that the 3.5" drive may be physically too big.
Also, aside from the measurements, are there any other differences that may cause compatibility or usability concerns?
The 2.5 and 3.5 are inches and they usually designate the diameter of the disk itself and not the housing. The enclosure will (of course) always be larger than that.
Here's a comparison (8, 5.25, 3.5, 2.5, 1.8 and 1 inch disk):
Both are able to run on different speeds, the faster, the better, although the notebook ones will mostly run on 5.400 rpm to save energy.
You can also use 2.5 in drives in a desktop PC, but I would suggest a dedicated mount so it doesn't vibrate.
Another difference which is worth mentioning is that since 2.5" drives are mostly used in mobile computers, main goal during design was low power consumption and other important factors were considered a lower priority.
Usually 2.5" drives have lower capacity, lower cache and lower angular velocity compared to 3.5" drives of similar price. For example at this time, largest consumer 3.5" drives can store up to 3 TB of data, may have as much as 64 MiB cache memory and are 7200 RPM or 10000 RPM. The downside is their power consumption which is generally around 5W. It is quite fine for desktop computers, but too much for battery operated devices.
On the other hand large 2.5" drives are in the 1 TB range and usually have between 8 MiB and 16 MiB of cache. The number of revolutions in minute is usually 5400, but may be as much as 7200. Their power consumption is lower and for example WD10TPVT by Western Digital uses up to 2.5 W.
While these numbers change with each generation of hard drives, one thing that more or less stays constant is the difference between performance of the drives and the price difference. Usually high performance 2.5" drive will cost as much as high performance 3.5" drive but will have performance of mid-range 3.5" drive.
Also 2.5" drives may be more resistant to vibration than 3.5" drives. For example some 2.5" drives have accelerometers which can detect when drive is dropped and safely part drive head in order to minimize vibration damage. Such systems are rarer among 3.5" drives, since desktop computers are usually designed not to be moved while working.
It is known as the form factor of the drive:
3.5": 4 in × 1 in × 5.75 in (101.6 mm × 25.4 mm × 146 mm)
2.5": 2.75 in × 0.275–0.59 in × 3.945 in (69.85 mm × 7–15 mm × 100 mm)
In addition to the 2.5" mobile drives, there are 2.5" "enterprise" drives for heavy file access. See the Seagate Savvio. By having smaller platters, high RPM is more practical, and seeks are physically shorter. Further, more drives can be packed into the same space, which, with a good RAID controller, will give higher performance.
Similarly, the Western Digital VelociRaptor drive uses smaller platters, even though the enclosure is the size of a standard 3.5" HDD, for greater performance, though with limited capacity.
It's the width of the drive (note that it's the actual platters, not the case, so the drives are slightly wider than this). These are standard drive sizes, and you should buy whichever one the device you're buying FOR supports. PC cases or laptops should blatantly say which size of drive bays are available, if you'll actually look. If you absolutely can't find a drive in the correct size, there are adapters and brackets to make small drives fit larger mounting as well.
As a rule of thumb, though, 3.5" tends to be a standard desktop hard drive, 2.5" tends to be solid state drives and laptop hard drives, and 1.8" tends to be laptop-only SSDs and HDDs. (5.25" is pretty much exclusively optical drives.)
As other answerers have indicated, 2.5" and 3.5" refer to the width of the drive platters. Most hard drives fall into these two sizes, and most have similar height and length within the same width category.
However, when looking to purchase drives that are of high capacity for their physical size (i.e.: 1+ TB for 2.5" drives), you need to watch out for non-standard drive sizes.
For example, the WD3200BPVT is a Western Digital 320 GB internal 2.5" drive. Its height is 9.5 mm, which is about normal for laptop drives. However, the WD10TPVT - a 1 TB internal 2.5" drive - has a height of 12.5 mm, and will not fit in many laptop hard drive bays.
If you are shopping for a high-capacity drive to go into a laptop today, you'll probably have a hard time finding anything far above 750 GB that will actually fit - let alone any high-speed ones. At this time, the highest-capacity Western Digital drive which runs at 7200 RPM and will fit in most laptops is the 750 GB WD7500BPKT.
**Note: I am not a Western Digital employee or advertiser, nor do I intend to portray Western Digital as the best hard drive manufacturer, or the maker of the biggest or fastest drives. I merely have a personal preference for their products, so it is to them that I generally look when shopping for hard drives and checking out current specifications.*
TL;DR: Always be sure to check with the hard drive manufacturer, and the manufacturer of whatever device the drive is intended for, to be sure that the new drive's physical dimensions will fit in the space available within your device. Even between drives of the same width (2.5", 3.5", or otherwise), other dimensions may vary.
3.5" when one says 3.5" drive, does not refer to the size of the drive. It refers to the so-called 3.5" Bay, so-called because it took a floppy disk drive that had a 3.5" disk! Diskette dimensions. The drive we identify as 3.5" but that comes from how we identify the bay which is historical based on the disks the drives in that bay took.
2.5" drives are called 2.5" drives because 2.5" is the diameter of the platter of the drive, I've heard.
BTW, the 5.25" bay, like the 3.5" bay, is named for the size of the diskette of the disk drive that occupied that bay. Now it's mainly used for DVD/CD Drives.
1.8" might be platter too,
http://en.wikipedia.org/wiki/Drive_bay
Also what size to get.. If it's not a laptop, then it's 3.5". And you should really have opened up your desktop and seen the place it'd go and recognise it.
If it's a laptop then check properly, 'cos there's 2.5" 12mm was old, 2.5" 9MM is common.. or was last time I checked. And some perhaps smaller ones use 1.8".. So a good idea to check the dimensions of the one in there. And then you'll know what size to get. (added- as of writing, they make 7mm laptop drives)
