"please insert a disc into drive D"

We just replaced 2 dvd drives in a Dell 8400. After doing so, we don't know
how to make our computer recognize the drives. Unfortunately that is why we
replaced them in the first place.... Anyway, when we try to use the "D"
drive we get the message "please insert a disc into drive D" even thought
there is a disc in the drive. According to the Device Manager, the device is
working properly. The computer does not now even recognize the "E" drive
which is identical to the "D" drive. The new drive(s) is listed as TSST CORP
CDDVDW-SH-S202J. There are no computer wizards here, not even close and it
looks like it was not neccessary to replace these drives even though the new
ones will be much faster after we solve this problem. We haven't yet even
been able to install the discs that came with the new drives, because of this
problem. If any one could take the time to help us with this we would be
very grateful. Thanks for your time. Anni