I have some older IDE (parallel ATA) hard drives I want to check for files before tossing them out. I have found SATA/IDE to USB Adapters to use, but they specify 2.5" and 3.5" hard drive connections. My drives are all 4" X 5 3/4".

My question is, are they considered to be 3.5" and is this based on the internal disk size? I don’t want to buy something I can’t use.

  • 2
    Note that PATA was invented when SATA came out. Before that it was only named IDE. Commented Apr 3, 2015 at 8:52
  • 2
    I think you mean "the name PATA was coined when SATA came out". +1 for trivia :)
    – jiggunjer
    Commented Apr 3, 2015 at 17:34
  • @ThorbjørnRavnAndersen Don't forget the distinction between IDE and EIDE.
    – user
    Commented Apr 3, 2015 at 17:58
  • No, ATA came after IDE. ATA in 1994, IDE in 1986.
    – JDługosz
    Commented Apr 4, 2015 at 10:14

4 Answers 4


The size of an internal-bay device is given by two figures: the "size", or form factor (usually 2.5", 3.5" or 5.25", but occasionally other values), and the height.

Wikipedia has a list of hard disk drive form factors. According to that table a 3.5" HDD is 146 mm long by 101.6 mm wide, or just under 5 3/4 inches by exactly 4 inches, and may be either 19 or 25.4 mm (just under 3/4 inches, or exactly 1 inch) high. Your HDD matches this and thus is said to be a "3.5-inch form factor" drive, or just 3.5" for short. As you can see, there is no direct relationship to any external measurement.

This is largely because the 3.5" and 5.25" form factor names trace their lineage back to the floppy disk sizes that were used with drives that had the specific form factor. For example, 3.5" floppy disks were 90 mm (about 3.54 inches) wide and were normally used in what came to be referred to as 3.5" form factor drives, which thus had to be slightly wider than 90 mm to safely and properly accomodate the media.

As for height, you commonly see half-height and approximately-quarter-height devices these days. A standard optical drive for mounting in a desktop computer is most often "half height", and this is generally the tallest single bay that modern computer cases allow for. Quarter height would be 20.6 mm high, which is slightly more than the slimmer 3.5" form factor. These terms compare to the original IBM PC floppy disk drive (here is an image of such a PC with two floppy disk drives), which was a "full height" device. Compare Wikipedia's article Drive bay and particularly this comparative image of different drive bay sizes. 2.5" drives are commonly much thinner than quarter height, and their heights are often just given in exact measurements instead. For example, the Intel 530 SSD is described by my online retailer of choice as a 2.5" form factor SATA device with the dimensions width 100.45 mm, height 7 mm and depth 69.85 mm. 2.5 inches is 63.5 mm; again, there is no direct relationship to any external measurement.

  • 3
    For 5.25" and 3.5" drives, the number actually refers to the size of the floppy disc that would fit in a drive of that size. That's why a 3.5" drive is really 4" wide; the drive has to be a bit wider than the floppy. Newer (smaller) drive sizes may refer to the maximum platter size possible.
    – cjm
    Commented Apr 2, 2015 at 19:08
  • @cjm Well, yes and no. 3.5" floppy disks were actually 90 mm wide and the magnetic disk pretty much filled the disk. That's approximately 3.54" wide human-handleable media.
    – user
    Commented Apr 2, 2015 at 19:55
  • Yes, but they were still called 3.5" discs, so that's what the corresponding drives were called.
    – cjm
    Commented Apr 2, 2015 at 19:58
  • @Federico “by” is the × in 5.75″ × 4″. It means 5.75″ length and 4″ width.
    – Daniel B
    Commented Apr 3, 2015 at 15:32

The important part here is really that desktop hard drives used a 40 pin connector with a 4 pin molex power supply connector. Laptops used 44 pins - which includes power.

A laptop PATA -> USB adaptor typically is self powered, or powered by a Y shaped USB connector with 2 USB A connectors for power

A desktop PATA -> USB adaptor typically has its own power supply and greater power needs.

Use the adaptor that matches the connector on your drive, rather than looking at the physical size.

  • Yup. One 44-pin connector: 2.5" drive. One 40-pin connector and a 4-pin Molex: 3.5" or 5.25" drive. (Did 5.25" IDEs even exist??) Commented Apr 2, 2015 at 20:34
  • @LorenPechtel ATAPI/PATA CD-ROM drives certainly did exist (also DVD drives and writers). I'm pretty sure I have one in a closet. Not sure about 5.25" PATA HDDs, but I don't see why not. That form factor lasted a fair while for HDDs and PATA was standardized reasonably early (mid-1980s, I think).
    – user
    Commented Apr 2, 2015 at 20:56
  • @MichaelKjörling Good point. 5.25" IDE opticals certainly did exist. The reason I question 5.25" PATA HDDs is I thought we were on 3.5" HDDs by the time PATA was on the scene. Maybe I just never encountered the hardware. Commented Apr 2, 2015 at 21:44
  • @LorenPechtel PATA was originally designed in 1986, says Wikipedia. Wikipedia also says that 5.25" HDDs were available in the late 1990s in both half-height and full-height form factors. While this doesn't prove that those drives used PATA, I'd say it's a strong bet that at least some did (although SCSI HDDs in 5.25" form factors almost certainly did exist as well).
    – user
    Commented Apr 2, 2015 at 22:05
  • @LorenPechtel, I've got a computer with a 5.25" quarter-height PATA/IDE hard drive, so yes, they do exist. They're just extremely rare.
    – Mark
    Commented Apr 2, 2015 at 23:14

The IDE connector is the same on 3.5" and 5.25" IDE hard disks, so as long as your USB adapter includes means to power the drive (it should, as 3.5" drives also require power separate from the IDE connector) it can be used to read both sizes.

The 2.5" connector is different, and includes a power connection to run the smaller laptop type drives.


I've seen PATA / USB adaptors in recent years have connectors on three edges and take anything. Reading old offline-storage drives is what they are for, and one cheap device is all that's needed. enter image description here

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