Since you were using Hardware Compression, I would imagine that the firmware levels on the two drives were different. I ran into this problem years ago with "identical" Compaq DLT4000s. Luckily I was just trying to balance out drive usage, and was able to restore from the original device
Since then I NEVER user HW cocmpression. With the system resource on computers these days, you will not see any throughput difference between the two and Vertias will always be alble to read Veritas compression. (Course if you are thinking of switching to ArcServer, stay away from Software Compression :D