Disappearing hard drives

My computer has 3 hard drives mounted - an SSD for windows and boot, and two WD500GB SATA drives; one for programs and data, and the other for backups.

Recently, the backup drive failed to be detected by the system. After basically jiggling the BIOS handles a few times, it reappeared ... but the program drive was now not present. I reordered the drives in the BIOS and then they were all back. Then the program drive was gone ... reboot and it is back but the backup is not.

Making matters worse, MS Office now requires reinstallation becasue it can't find components to operate or repair. Gah!

Not overclocked, no customizations in the bios. Computer has been running essentially as-is for years. WTF?

What BIOS' handles?

Another reason to never separate programs from the OS drive, where they write registry keys and should remain until properly uninstalled.

Test the cables and connections.

Test the hard drives with the maker's HD Diagnostic extended CD scan followed by a full Disk Check.

Test the cables and connections.

After doing this, load defaults in BIOS (either first screen or exit menu).

Then check if drive lettering has changed with all the disappearing.

Following up here;
Drives still come and go seemingly at random on cold boot and presumably this causes the hard drive letter and boot order changes. The boot drive is the SSD and that tests OK. WD diagnostics say one of the WD Caviar Blue 500GB has "recoverable" bad sectors. Knowing how that movie plays out, I'll be replacing at least that drive.

Not too cheerful with WD since the warranty on that drive expired just about 40 days ago. Precision manufacturing!

Anyway, I still don't understand how some bad sectors on a HDD (with no apparent data errors) can produce the issues I'm having, especially when the 'bad' drive is the one that actually loads up most of the time.

Remove the faulty drive and see if it corrects the issue.

And I would still ask WD/distributer for a possible replacement. Being nice pays off sometimes :)