How to check HDD health (before SMART drives)?

[Jope]

Fill it with data you know, read it all back. If there are no read/write errors and the checksums match, the drive still works. No-one will know how long it will continue working, however. :-)

[Jope]

Thanks, I'll look into this. Did you use this before SMART became a thing?

No, before that we just made regular backups (haha, no cash strapped teenage hobbyist did, but we knew we should have) and hoped the drive doesn't die.

Otoh in the Amiga times most hard disk owners had a lot of their less actively used data on floppies too, thanks to the low price of floppies and small capacity of hard drives.