The first hard drive for an IBM compatible, around 1982 or 83, was a full height (3.25" high by 5.75" wide) monster with eight disk & 16 heads. It was all of a whopping 5 MB. The disk rotated at a rate of 1,800 RPM. The throughput was so slow that you probably could not load Windows on it today (even if it was large enough). Average access time was something like 220 ms. It cost only $1,800.00... discounted.
If that was not bad enough, you had to manually set up the drive in the BIOS as they had no automatic detection as do today's drives. That was a real mess & not for the faint of heart.
Today's 3.25" HD's have one or two disk, 2 - 4 heads, are 1" high, 3.25" wide, are up to 120 - 300 MB, rotate at 7,200 or 10,000 RPM, average access time of less then 9 ms & cost less then $200.00. Plus, all drives now (for the last 10 years or so) have automatic BIOS detection done with a unique drive identifier chip in each drive model that tells the BIOS what the setting should be for that model drive... a real life saver indeed.
The last Maxtor 120 GB, 7200 RPM, 8 MB cache drive I bought cost only $93.00.
If Seagate has their way, the next big deal in HD's will be a gradual switch away from the 3.5" format to the 2.5" drives.
They are already trying to convince the server industry to switch as the 2.5" drive has a lot of advantages over the 3.5" format, mainly speed. There is a limit of around 12,000 - 15,000 sustained RPM for 3.5" drives before the disk shatters from centrifugal force & general disk failure due to fractures in the plater. 2.5" drives can sustain much higher speeds due to the smaller mass of the platter.
With the high aerial densities of today's disk & about 10X densities to come, the 2.5" format seems like the next logical transition for computers. The combination of much higher aerial densities along with 2 -3 times the disk speed will provide drives 4 - 6 times faster then what we now have.
And, there is more to come in the near future.
Man oh man, how I LOVE technology