NetWare 6.5 data pools are normally compatible for clustering purposes with NetWare 6.0, both backwards and forwards. The Media Manager changed between NetWare 6.0 and 6.5, but the media format is the same.
If you move a device from the NetWare 6.5 server to a NetWare 6.0 server, and if you can see the NetWare 6.5 pool on the disk, but cannot activate it on your NetWare 6.0 server, the most likely causes might be a name conflict or that the pool was mirrored.
Pool and volume names must be unique on the server. Renaming the NetWare 6.5 pool should allow it to be activated on the NetWare 6 server.
If the pool was mirrored, the old media manager on NetWare 6.0 probably would have problems with it because the information is different. You should remove the mirror before trying to move the disk to NetWare 6.0.