Seen few bad disk controllers - dying chipsets could do this mess. Its rare, but it happens sometimes. Dunno how to check it on NB. At desktop it was easy - use addon onboard or PCI ide/sata controller.

That is odd, since it is passing those stress tests. You could try Prime95 but I'm guessing it would probably pass also.
You've replaced the hard drive yet still get the same symptoms. If you're completely sure at least one of those drives is definitely good, then it seems it could be a disk controller problem on the motherboard. Strange that it just suddenly started doing that though.

Back in the 486 days, I had a bad drive controller card which would intermittently have problems seeing the attached drive and would freeze at boot. Later, I saw the same behavior on a faulty P3 Thinkpad motherboard, and at the time I thought it's symptoms matched that old 486's behavior. However, on those machines, I think once they booted they were generally fine, I don't remember them corrupting a filesystem.

I don't know if there's any chance power management could be a factor, but I guess I'd try disabling all such features and see if that makes any difference.
Are you using a known good power supply with the laptop? I tried a knockoff PSU on a Thinkpad one time (different unit than above) and it caused instability.

Another unlikely guess, but is there any chance the disk isn't making good contact with the motherboard?
This seemed to be a problem on an older Compaq LTE5380 laptop. It caused slow disk performance but I never noticed corruption - presumably there was some error detection involved which avoided errors but slowed it down. It was also running NTFS so I'm not sure the errors would have been as easily seen as with FAT32. The drive caddy needed to be reseated periodically to temporarily fix it.

I've installed Win2000 on it now just to see if the problem is isolated to 98, so I'll install Prime95 and leave it going for a while.

The power supply is the original Toshiba one which has always worked up until now, so I assume it's still good.

Last night I pulled the whole laptop apart just to see if anything was shorting out or not making proper contact. It all looked fine, but at least now everything has been reseated so the connections should be good.

Interestingly it seems to be corrupting parts of the drive that it shouldn't even be writing to. Last night it had the same problem again, so I booted off a CD and ran fdisk to repartition the drive, but the drive label for C: was all random symbols. Clearly I hadn't set it to that, plus it made it impossible to delete the partition using fdisk as it makes you type the partition name for confirmation.

The laptop has 320MB RAM, and I seem to recall back in the day people saying that too much memory on Win9x could cause instability. It always used to have 320MB and ran fine, but could this be a problem?

Windows 9x had a bug where, on computers with very fast processors (Faster than ~500MHz IIRC), it would shut down quicker than the hard drive could flush its cache, effectively corrupting small bits of data at a time. Depending on what exactly was being written at the time the drive loses power, this could cause all sorts of random issues. A hallmark symptom of this bug was Scandisk automatically running at every bootup even if it had otherwise been shut down normally.

Windows 2000 does not have this bug, so if this is indeed the cause of your corruption, then upgrading to Windows 2000 will have solved it. If you wish to roll back to Win98, you'll have to install an update (KB273017), and then you MAY need to also edit the registry.

See https://support.microsoft.com/en-us/kb/273017 for instructions on how to edit the registry. The update itself is no longer available for download on Microsoft's website though, so you'll have to find it somewhere else. I used to have a copy, but don't know if I still do. If I find it, I'll put it somewhere and post a link to it here. Note that there are two versions of this update available, one for Win98 and the other for WinME. If you do decide to try finding it yourself, you'll have to make certain it's the version for Win98. Otherwise, hopefully someone else here has a copy they can make available for you.

seanneko wrote:

Interestingly it seems to be corrupting parts of the drive that it shouldn't even be writing to. Last night it had the same problem again, so I booted off a CD and ran fdisk to repartition the drive, but the drive label for C: was all random symbols. Clearly I hadn't set it to that, plus it made it impossible to delete the partition using fdisk as it makes you type the partition name for confirmation.

Any version of Linux should include a version of fdisk with no such requirement.

The laptop has 320MB RAM, and I seem to recall back in the day people saying that too much memory on Win9x could cause instability. It always used to have 320MB and ran fine, but could this be a problem?

The RAM problems in Win9x kick in at 512 MB. Even then, it just makes Windows crash; it certainly doesn't wreck the filesystem. Besides, as you were saying, it was running fine with no issues.

Have you checked the RAM?

Well, I've copied about 30GB of files to the Win2k installation and run Prime95 for several hours, and it seems okay... There might be some hidden corruption that hasn't caused the OS to crash though. All I can say is that it's still running and appears normal.

Not sure what to think right now. I know for a fact that it used to run Win98 completely stable. I'm going to burn a new install CD just in case the old one had any problems and give it another shot. If it still has the same issue then I might have to leave Win2k on it longer term to see what happens after days/weeks.

Run antivirus: sometimes that is the cause.

Okay, so it gets stranger.

I've verified that the ISO I'm installing from is an original from MSDN (compared the SHA1 checksum). I've burnt it a couple of times just to make sure I didn't have a faulty CD. The problem happens even without me installing any drivers, so it's not a bad device driver causing issues.

At this point I was pretty much convinced that it was a hardware issue and the laptop is dying, but then I installed Windows 98 on a Thinkpad 390X I have here and it's having the exact same problem! I'm totally out of ideas here. The chances of two of my laptops both having the exact same hardware failure is pretty slim.

I've installed Windows ME (which is what the Toshiba originally came with when new) so I'll see how that goes over the next few days.

then I installed Windows 98 on a Thinkpad 390X I have here and it's having the exact same problem!

Are you saying the problems started happening immediately after you installed Windows 98? Or did you do something else in addition to using the install CD?

seanneko wrote:

I've installed Windows ME (which is what the Toshiba originally came with when new) so I'll see how that goes over the next few days.

Just make sure you disable system restore first because it's bugged after some point in early 2001 unpatched.

Jorpho wrote:

Are you saying the problems started happening immediately after you installed Windows 98? Or did you do something else in addition to using the install CD?

I'm pretty sure I installed drivers on the Thinkpad (video, audio, etc), but these are all from IBM and the laptop does officially support 98. The only common factor between the two laptops is that they both have Xircom RBEM-56G network cards and they were both installed from the same install CD.

leileilol wrote:

Just make sure you disable system restore first because it's bugged after some point in early 2001 unpatched.

I disable system restore even on my new computers 😀 I wasn't aware that it had a bug in ME though, thanks.

torindkflt wrote:

See https://support.microsoft.com/en-us/kb/273017 for instructions on how to edit the registry. The update itself is no longer available for download on Microsoft's website though, so you'll have to find it somewhere else. I used to have a copy, but don't know if I still do. If I find it, I'll put it somewhere and post a link to it here. Note that there are two versions of this update available, one for Win98 and the other for WinME. If you do decide to try finding it yourself, you'll have to make certain it's the version for Win98. Otherwise, hopefully someone else here has a copy they can make available for you.

The update itself, for Win98SE, is still available on Vogons, linked in this post:
Re: Issues With My Old Windows 98

I've had the same problem on a desktop PC - Pentium 3 which I got for a few Euros.

The problem was that the user before had flashed the wrong BIOS.

After flashing with correct BIOS it works.

And on my Abit KT7 Board, on IDE controller is half dead...
when I use the primary controller always gets errors and the system falls down to PIO mode.

So I connect CD-ROM to primary and HDDs to secondary.

And on a P4 HT machine, Windows XP destroys itself after a few days.
The problem was the combination of motherboard and HDD.
The HDD found errors with surface scan when formatted FAT32, in NTFS surface scan works but Windows destroys itself after a few days.

The motherboard works great now with another HDD.
And the HDD from this PC now works error free in another PC.

Can you try setting the HDD in the BIOS of the Laptop to not use UDMA and use only PIO?

seanneko wrote:

I'm pretty sure I installed drivers on the Thinkpad (video, audio, etc), but these are all from IBM and the laptop does officially support 98.

Were you using an official drivers disk? It is conceivable (though unlikely) that a virus got into something you prepared personally.