Do you know if any of them have bad sectors? Have you even tried chkdsk (windows) or fsck (linux)? Spend 10 minutes checking, then tell us if they have bad sectors. Assuming can be bad sometimes.
Hard drives in a desktop or some other solid place last for a very long time. I still have a 7 GB HD from a Dell Dimension V400 (cira late 90's) that still works just fine (although slowly). Hard drives are also rated for hundreds of thousands of hours, or several years of constant use, more if not used so often.
Since you have a shiny new 1 TB HD, why not use the old one's for backup? RAID isn't an option and trying to manually rotate drives can be hard, but you can use a spanned volume where one partition spans multiple hard drives. Since they are backups, if one fails then you can just recreate the spanned volume (this is obviously not for critical files).
Honestly though, 2 years of usage for a hard drive is not long. You don't have anything to worry about for a few more years.
1In practice, hard drives can last almost indefinitely. That is to say, the other end of the bathtub curve is usually more than a decade away from the purchase date. – marcusw – 2010-08-10T15:33:55.047
@marcusw: I would say in theory, in practice hard drives take damage from falling (unless the hard drive is protected against that, mine has fell from shoulder high and still works perfectly), damage from power spikes (because power supplies can malfunction but are not directly seen as the cause), damage from reading a lot of fragmented data (solved by defragmenting), etc... – Tamara Wijsman – 2010-08-15T19:56:38.180