If you were using HW compression, you may be SOL
I've been told multiple times by various people that (at least for DDS) HW compression should be compatible across manufacturers, but I got 7000with an older firmware than the COMPAQ DLT7000 that made the backup. Luckily, I still had the original lying around and was able to attach it to one of my backup servers and get the data restored..
Since then I've never used HW compression again. On today's server hardware, SW compression runs just as fast, and will never have that problem.