POH is a good indicator, but not the only factor. A good way to keep old disks healthy is to read from/write to each sector on some regular schedule. That will "refresh" the sectors and keep what some people call "bit rot" from setting in. There are various ways of doing this, some destructive, some non-destructive:
-DBAN the drive (destructive)
-use something like SpinRite or DiskFresh (non-destructive)
-running full defrags is better than nothing, but not guaranteed to hit every sector.

SpinRite has been the best tool for me as it has various levels to exercise the sectors more or less. It also has a means of recovering data in bad sectors. It's a good maintenance tool and is flexible enough to run from a floppy so you don't have to move old disks into new computers to run it.

The problem is that it's usually impossible to know the past history of those drives, especially until they started incorporating SMART which at least allows you to know the total power-on hours. So there's no way to tell whether the drive has been running 24/7 for years or just used sparingly, how many different owners it had or if it ran in a hot/dusty case. All of those are factors that can impact the life expectancy of the drive.

My #1 rule regarding used HDDs is to not pay a single cent for those, all the ones I have were salvaged from PCs and if they pass a surface test I use them until they die. When they fail, I just discard them.

This is something every vintage user will experience sooner or later. Old disks just become unreliable. Although I have only had 5 disk failures in my including 2 mfm drives and an XTA. This last week one drive blew a bearing... Can't prevent that. The hot weather we have in the Netherlands at the moment doesn't really help as well.

I've had about 300 drives total, so it's a low failure rate... And I don't trust any. Still buying new old ones, hoping they last another age.

For important stuff on your discs there is the backup, for other stuff there's a reinstall.
For hardware, there are spares, with compactflash as final station.

I do spin up my hard drives every one and a while, even spares. This keeps the lubricants in shape and hopefully prevents stiction.
Every once and a while a fresh install after a full format helps as well, or "erasing empty space" single pass with data destruction tools of you want to keep the installs and software.

I had 3 SATA hard drives and an SSD, all just out of warranty, fail on me within a 6 month period last year. They are the only drive failures I have had in 27 years since building my first PC.

I now have about 40 scrap ATA and SATA drives from about 400MB to 1TB to test, and expect half to fail. Should give me enough for a bunch of retro builds I'm planning.

Thanks for the answers. Considering this machine I'm talking about is a 386DX based one, there's no problem on formatting and reinstalling everything just it's sad to know that sooner or later an app or some data will end up raising some error cause written on problematic clusters. And than begin again.. scandisk.. defrag.. scandisk etc.. and after a while not really trust the disk and begin from zero again.
Considering I'm looking for different size of disks but I'd like to stay into the msdos/bios limits of 1024 cylinders, which were the usual MB/Mb sizes they were usually built without searching for the exact models codes in the 486/early Pentium era?

Considering 386 limits, the magical C1024 H16 S63 limits produce the 528MB limit.
I have yet to see a hard disk that exceeds the values without getting above these figures.

There are a lot of hard disks between 528MB and about 1GB that support a 1024 cilinder mode, to support operation with these limits.

There are even some disks (like my WDC 850MB) that support a dual disk mode. This phisically reports itself as master and slave drive on the IDE channel, both half the total capacity. This does however rule out the possibility to use a physical slave drive.

That said: At the moment my 386 runs a ~800MB Seagate Decathlon as 528MB. A 2GB drive works just the same.

I also found most (all?) 512MB compactflash cards fit in these limits. I have some Transcend industrial ones, a Kingston and a SanDisk.

I think the "expected" lifespan of a hard drive is only a few years, so it's amazing to me any of these old hard drives still work after 20 or 30 years.

I wouldn't expect much from drives sold by scrappers.
A recycling station looks like this: huge piles of different hardware on the floor, thrown around like... e-waste.
The better ones are selected for retro business, the rest gets recycled.
By the time you buy them many of them are dead (they call it untested).
If a drive (or any other part) is heavily scratched then it comes from a recycling station.

Vintage HDDs tend to be a crapshoot in general. It'd help if one were to know the history of a drive that's sitting on the shelf.
Very well treated or barely-used drives have a *generally* higher chance of working than one that seems like it was tossed around the room.
I had to deep-six a few Conners earlier that were reaching the end of their useful lives.
That said, I'm running a handful of rather well-treated Quantum EX drives on my 'tweener' systems, and they don't even have loud spindles.

chublord wrote on 2020-08-09, 19:08:

I think the "expected" lifespan of a hard drive is only a few years, so it's amazing to me any of these old hard drives still work after 20 or 30 years.

Manufacturers usually specify life expectancy assuming the drive is going to be running all the time and at a certain temperature above ambient during its entire lifetime, which is not always the case. That's why a lowly IDE drive that has not seen much use (or none at all) can work just fine (assuming no bit rot and that the bearings haven't dried up) while a server-grade SCSI HDD with a MTBF of 200,000h that's been going on non-stop since the 90s is more likely to fail at any moment.

chublord wrote on 2020-08-09, 19:08:

I think the "expected" lifespan of a hard drive is only a few years, so it's amazing to me any of these old hard drives still work after 20 or 30 years.

Typically it's 50,000 POH or more, which equates to 5+ years of 24/7 use. I have a 6.4GB Fireball CR that has 6623 POH and so far it's been completely error free. It is a crapshoot, but if the drive has lots of POH, it's more likely to go bad than even a very old drive with few POH.

I agree about the POH as being the main factor as to how long a drive will last, abuse and on/off cycles being combined as next. If you fire up a old HD make sure you run it long enough to get up to operating temperature before shutting it down, most do some re-calibration during warm up and it helps keep the bearings from failing versus just turned on a few minutes. With newer SMART at least you can check but old drives prior you cannot which is a real bummer for all of us, so hard to tell how many hours a certain drive may have other than seeing just a bunch files dated over a few years (I always look at existing data dates on any non smart HD that I get). Yes ! Spinrite is a good tool for testing older HDs. Amazing that I have some boxes of old IDE and SCSI from 40MB thru 3Gb that still work perfectly, though in last 10 years had 5 die out of 30 🙁 .

clueless1 wrote on 2020-08-09, 22:01:

I have a 6.4GB Fireball CR that has 6623 POH and so far it's been completely error free.

That is great! With care it could easily last a few more decades. I have a WD 500Gb from 2015 with 1310 and another 500Gb from 2015 with 18,500 hours, both bought used and cheap. Just goes to show that you can get a barely used HD and a very used HD from same time frame and only a good tech will know the difference between them....
I generally use Crystal DiskInfo to check the SMART on my HD's, if some some one has a better Windows app please let tell !

Also feel like drive overlay or not using the full disk is, um cheating?
So have a physical drive as the primary but mostly only has Dos/Win3x on it so can can be relatively small drive.
I get authentic sounds during boot and disk speed isn't really that important.

I then have the largest supported CF card for games and data where the faster disk speed is more beneficial and you don't notice the lack of sound.
If that 1 CF card still isn't big enough you can easily swap them out "cartridge style" if all are the same type.

Just an idea to open up more options, IDE drives especially desirable sizes like 500 MB and 8GB are only going to get increasingly rare.

I've lost count of the number of HDDs in my collection, but failure rates are pretty low - however most aren't used on a regular basis so probably haven't accrued a lot of power on hours. The worst drives for failure rates for me were the newer 3TB SATA drives, those 3TB drives should really be avoided like the plague!

My hardiest HDD to-date is a 200GB Maxtor IDE drive that I bought back in 2000. That thing has been running near 24/7 for the past 20 years in 1 system or another. It started out life in a Pentium III PC and currently lives in a Core 2 Quad PC. Whenever I upgraded to a new PC for family usage, that drive followed. It has 4 reallocated sectors & 2 uncorrectable - but that has been like that for the better part of a decade and shows no signs of worsening!

The other strong HDD in my collection is a Seagate 20MB MFM HDD. It survived the 80s in an XT used in a local youth center, before I got it in the early 90s and eventually retired it around 1996/7. It then got knocked about in storage quite a bit, with it's front plastic besel snapping off etc. I dug it out again around 2017/2018, low level formatted it and it's still going strong in my current 8088 build today. They cetainly don't make them like that anymore!

In saying all that though, I wouldn't bother trying to purchase old IDE or MFM drives anymore. It's either IDE-CF or IDE-SATA for me nowadays, much easier availability vs reliability.

Its like a Vintage car. It needs TLC. Old used drives will die sooner or later. It should NOT be used as a critical everyday computer for storing your data. Its for Vintage game play Like an old arcade machine. But that being said. I have never had a old drive below 1gb go bad on me.

Try Nortons disk doctor works great for old Mac’s too.
https://winworldpc.com/product/norton-utilities/20x

I like the Nostalgic sound of a mechanical hard drive.

You can still find NOS IDE hard-drives on eBay.

I have one of those old 20 MB Seagates. Works well, but it makes a terrible squealing noise. How to fix this?

That sounds like one of the bearings is needing some oil. You can try and see if you can service the bearings. I remember I had a mfm drive that had oiling ports.

What oil is used for this? Sewing machine oil?

Back when I was a kid I remember my dad using 3 in 1 oil once a month when he cleaned and dusted the computer