The drive may be being identified during bootup but that may only mean
the information about make, model, capacity size etc are being read by
your BIOS from the chip containing that information on the hard drive.
That particular section may still be functioning electrically on the
hard drive but that's no guarantee that other circuits, eg, controller,
motor etc, are operational.

Bottom line is, if the motor isn't spinning to turn the hard drive's
platters, the heads crashed into the platters or the controller doesn't
move the heads, the drives are toast......which would make sense since
Dell figured you had good reason for replacing the hard drives.

A data recovery company could probably retrieve the information but
recovery services are not cheap...especially when on board electronics
are toasted as that would mean having to rebuild/repair the drive.

Some hard drive manufacturers have hard drive testing programs available
for downloading on their sites......you might want to check out to see
if such a program is available for your units.  They won't be able to
fix the drives but will give you a definite answer as to state the
drives are functioning.

You could also try using a emergency boot up disk, like the one
available from the following location:

http://www.sysresccd.org/Main_Page

The particular version is for Linux installations but can also read
Windows formatted hard disks.  Probably similar Windows versions are
available but off-hand I am not able to give you any specifics.

Good luck.  :-)

If you still have RAID enabled in the BIOS, you might need to install the
RAID driver in Vista to see the drive.

I'm happy to report that I used BIOS to reconfigure the drives to non-raid,
and I was then able to boot from one of the drives, despite BIOS previously
reporting the drive as failed. Yay!