The next is a totally out-of-nowhere conceived step-list. Its still work in progress, any suggestion can be made and will be podnerated and if good enough added to the list.
If you find some DAT Tapes, the steps to follow is
a) check theyr condition
b) track down the model
c) retrieve a tape drive
c1) try to find out which software has been used to backup on the tape (TapeWare etc)
d) get data from the tape
Well, plain and simple, eh? But the hard part comes with steps c) and d).
Retrieving a suitable drive is usually complex. Especially if you know nothing about backup activities (like me). Wikipedia offers some insight into compatibility and formats, but still there is no guidance or howto's around that i know of (if someone knows of them, please reply with a link).
Usually bear in mind that tape drives can only read equal or less capacity tapes (that is a 2 GB tape drive wont be able to read a 3 GB one).
Also, perhaps the biggest of the harder issues is the d1 - to get ahold of the software that has been used for backing up. The simple Windows back up package is quite common, but it's not the standard. So a first guess after you got your drive would be to try with it.
"And what about unix then?" Good question. Have no idea. Suggestions?
Or leave it all to the experts, and pay : http://www.ontrackdatarecovery.com/