When people talk about 2.5, 3.5, and 5.25 -inch drives, are they referring to the longer dimension of the drive, the shorter dimension of the drive, or the diameter of the actual disk inside the drive?
They're talking about the size of the platters inside the drive. The actual drive cases are about a half-inch wider than the platters and about 50% longer in the other dimension to leave room for the head actuator.
Just out of curiosity, where would 5.25" hard drives would be used? My desktop computer only has the 3.5" size, so I'm wondering if I would ever run into any 5.25" hard drives.
The 5.25 HDD's are actually the size of the container, the size of the physical drive, and was named from the fact that they were designed to fit into a 5.25" opening full height in the front on your computer, just like a DVD now fits in. Now most full sized HDD's fit in a HDD cage inside a desktop computer, but they can be attached (directly or with plastic rales on each side like Dell uses) in the front of your desktop with the face plate put back on so you don't see anything. That's the common size you see advertised as regular HDD's
When Sminlal was nicely describing the 3 1/2" drive platters, they most commonly are inside a 5.25" HDD physical case. The 2.5" drives are pretty much used for laptop computer HDD's, and external HDD's that get their power from the connecting USB cable.
When Sminlal was nicely describing the 3 1/2" drive platters, they most commonly are inside a 5.25" HDD physical case.
Actually today's "desktop" 3.5" drives fit inside a smaller case that's only about 4" wide by 6" long. But before that size came long there actually were drives with 5.25" platters - they fit into a case that was about 5.5 inches wide - see this Wikipedia entry and this picture:
The drive at the bottom left uses 5.25" platters, the one immediately to it's right is a 3.5"-platter drive of the type that is today called a "desktop" drive.
5.25" hard drives are no longer made, but desktop DVD drives still use the same form factor that those drives used to use - so if you could find one of those old 5.25" drives you'd be able to screw it into a DVD drive bay. Well, you could screw in the "newest" of those old drives - the really old ones were twice as high as a standard DVD drive.
That's a wonderful photo of the different sizes of HDD's! Very nice & worth saving next time the question comes up..
Remember the first IBM 10 MB HDD in about 1984 that took up 2 device slots and cost near $1000. Could that be the largest one in your photo?
Not quite, but maybe really close. The largest one in the photo has a sticker on it that says "20 MB MFM". MFM was the data encoding system used in early Seagate ST506 HDD units. But the first HDD's that IBM put in their PC-XT models was 10 MB. 20 MB and 30 MB came a little later, spurred in part by that upstart competitor Compaq whose clone machines offered them as one of their advantages over the IBM design.
By the way, 5¼" is the width of the slot in a floppy disk drive for 5¼" diskettes - I just measured on old diskette and it is slightly less than that. The actual mounting slot in a PC case is nearly 5¾" wide - I also measured an old backup tape drive. (I'm beginning to feel like a museum curator!) Created to accommodate those devices, the slot size has been retained and used for many others over the last 4 decades. However, as Sminlal says, many of the early HDD's were twice as high as today's slots, so what we use now are called "half-height" drive mounting bays.
Small aside: has anyone else ever seen the precursors, the 8" floppy drives and diskettes?
Paperdoc's correct - the 5.25" drive in my photo is a "newer" half-height drive, not one of the original IBM AT drives. I actually do have a couple of full-height 5.25" hard drives, but neither is from an original IBM AT. I also have an HDA with 14" platters from a mainframe-era Storagetek drive. In the photo below the small 3.5" Western Digital drive you see beside it can hold almost 1,000 times more data:
I used 8" floppies quite a lot in the old days when my company had a pair of DEC VAX-11/780 computers. The software for their PDP11 console processors was stored on those floppies, and I still have a few of them. Here's a shot comparing the sizes of floppy disks:
I've never seen that Storagetek HDD unit before. I have seen the old IBM System 360 mainframe HDD units that had replaceable HDD disk sets on spindles. They looked like a stack of about ten 12" or 14" vinyl LP's on a single spindle, and that entire assembly was lowered into a drive unit nearly the size of an automatic dishwasher.
The open appearance of that unit and the one I described may shock some people. Yes, they really were open to surrounding air! The secret is they were NOT Winchester designs. All today's HDD's are Winchesters. They have heads mounted on semi-flexible arms. When the disk is at rest the heads actually rest on the disk surface. But when the disks are turning, a thin film of air carried along by the disk surface lifts the heads microscopically up and they do not touch anything. However, the gap is so small, any dust particle could get trapped under the head and gouge the surface, ruining both disk surface and head. So all Winchester disk deigns must be enclosed in a dust-free sealed container.
Those older drive units sminlal and I talk about were open, and the heads were on rigid arms that maintained a fixed clearance from the disk surface. The magnetic fields had to be much stronger to work over these larger clearances, so the data density was much lower, the data transfer rate much smaller, and the total capacity of the multi-disk unit much lower than later sealed Winchester designs.
The open appearance of that unit and the one I described may shock some people.
To be fair, the Storagetek Drive in my photo was not normally opened like that - I just removed the top cover so that you could see the actual platters. But it also wasn't factory-sealed like modern drives are - even with the cover on air can still move freely into or out of the interior by flowing past the actuator arms (which hold the copper coils you can see on the left side). When the "HDA" (head-disk assembly) was installed it was hooked up to pressurized clean air lines through ports built into the case and it would be put through a "purge" cycle for over an hour to scavenge all of the dust from the air before the heads were loaded.
I have seen the old IBM System 360 mainframe HDD units that had replaceable HDD disk sets on spindles. They looked like a stack of about ten 12" or 14" vinyl LP's on a single spindle, and that entire assembly was lowered into a drive unit nearly the size of an automatic dishwasher.
Let me pull another picture out of my hat. Unlike the ones above, I don't actually have the unit pictured below - this is just a photo I took back in the days that I worked with them. The unit in question is an IBM 2311 which holds 7.5MBytes on a removable disk "pack". It has six 14" platters with 10 heads that record on all of the platter surfaces except the very top and very bottom. So each surface of one of the platters holds less than 1 megabyte. The windows in the drive are part of the "door" that you flip up in order to swap disk "packs".
We did have an 8" floppy unit in our office in the early 80's. Ran CP/M!
Brings back a lot of fond memories. The one memory and story of that Control Program for Microcomputers system was about Gary Kindall in Monterey, CA. who developed CP/M. We are just So. of San Francisco, and the story here was IBM initially went to Dr. Kindall to license his OS in their development of the PC, but for some reason, I think a non disclosure agreement his wife didn't like, it didn't work out so IBM flew up to Seattle to visit Mr. Gates, and that was the beginning of MSDOS and the revolution. And CP/M dwindled and so did their floppys.