Can't view differential mailbox items with only diff drive in

Hi,

Trying to figure something out.

In a particular configuration, we have a server with RDX1000 drives.  We use a 1.5TB drive for full backups and switch to a 500gb for daily differentials.

When trying to restore mailbox items from a differential job, mailbox items in Restore says 'None.'

However, if we allow the differential backup job to be performed on the same (1.5tb) RDX containing the full backup, we can view the mailbox items in Restore from the differential job.

Why can we not view mailbox items from the differential job while only the dedicated differential RDX is inserted?

 
Thanks in advance!