$28 hard drive from Amazon turns out to be legit despite the price — questions about the 500GB HDD's longevity remain, though

In the golden days of stepper motor hard drives, there would have been no limit on the media, as long as it didn't physically deteriorate e.g. due to exposure.

And you could always do a hard format which counted the stepper impulses and used the equivalent of the floppy disk hole to reset a sector timer: it wrote both sector headers and user data much like a floppy did, physical bit by physical bit in FM, MFM or RLL encodings, no pre-formatting required. The disk drives in those days were also truly dumb, all the intelligence was on the disk controller, which was an add-in card. If your electronics had survived a neutron bomb and you could reload your DOS from punch cards, ticker tape or similar, you could always reformat a black hard drive and start anew, not that I tried that.

With servo drives and heads there was nothing mechanical to fall back on, so track and sector positioning information had to be written onto the medium at production, also magnetically, of course, and only sector content data was ever overwritten, with a smart disk controller inside the disk drive ensuring that. Magnetic decay or external fields would result in an unusable drive, even if the medium and the electronic remained in perfect state.

Because you can't rewrite the servo information and a "hard format" was no longer that, only a read-check with defect management.

But then that magnetic decay didn't happen on its own? While a netron bomb was sure to wipe out everything in the wrong place?

In any case, a hard drive at rest with working seals, safely stored and with electronics not harmed by age, should last very long indeed, with eletronics, firmware storage, and the fluid in the ball bearings being more prone to decay than the magnetic encodings.

And I've only recently retired a RAID made from 2TB disks made around 2008. It was a 24x7 primary for years before it became a backup, so those drives certainly earned their original price. I'm sure they'd still work for many more years, it's only when you hold an 8TB NVMe stick in the palm of your hand (or a 36TB HDD), that you decide it's over for them.

I no longer take them apart, but I do remember how interesting it was on the first ones (fortunately company owned).

I distinctly remember a 30MB 5 1/4" full-height Tandon drive with a straight voice coil servo (first non-stepper HDD I ever saw) sometime in the 1980's and used in an IBM PC-XT, which had stopped working all of a sudden, just made odd clicky noises.

When I took it apart the reason became clear: the top head had detached from the head carrier and was hanging on the connecting wire. And since that particular drive used the top surface of the top platter to exclusively hold the servo information, while all other heads and platters held only the drive data, it could no longer position anywhere on the drive and all data was inaccessible.

That servo was moving a whole sled with the entire head and carrier assembly forth and back with a significant mass and with access times of perhaps 30ms, which was much lower than the 100-150ms that stepper motors could do at the time.

You wouldn't have wanted your finger in there while it was operating and you could feel the seek action on the entire table-top.

Of course later drives not only reduced the head assembly mass by switching to rotary positioning, but they also switched to embedded servo information on each platter side. Mechanical tolerances and temperature expansion would cover thousands of tracks by today's density standards, it's 30TB not 30MB in a much smaller form factor.