From the BE 2012 admin guide, under the Exchange section:
Configure a backup job that has all of the following settings:
- A disk storage device other than a legacy backup-to-disk folder is the destination device.
- The Granular Recovery Technology option is enabled.
- A backup method other than a snapshot method.
Another way of seeing this is that for GRT jobs, a disk storage device should be used.
The technical reason behind this requirement is that the graunlar items of a backup could only be mapped and recorded in to the catalog (so you could see them during restore) from a random access device.
Disks are random access devices, tapes are sequential access devices.
That also explains why when you first backup to disk, then duplicate to tape, you would still have the graunlar information in the catalog for you to see and restore granular items with, from tape.
When a backup first arrives at disk, graunlar information will be collected from it and into the catalog, before the backup gets duplicated to tape.
(Although in this case you would still have to first duplicate from tape back to disk before a restore because of the same random access requirement.)
If you backup directly to tape without first going through disk, then the catalog won't be able to collect/map the granular components from the backup. Hence, you will not see granular items during restore; you will only be able restore the entire backup as a whole.
If a backup goes directly to tape, I don't think duplicating it to disk would enable granular items to be restored because if the granular mapping isn't done during the initial backup, it couldn't be done later.
