Not sure about Firewire, but with USB, if you remove the device, it must be mounted at the same drive letter when re-attaching
Also, did you specify a standard B2D folder when creating this, or a B2D on Removable Media?
Depending on your Global Overwrite option, BackupExec may not accept "imported" media as overwritable. try setting this to "partial" rather than "full"
(Don't know why it would be imported if everything is being done on one media server - maybe see my first paragraph?)