We we expanding this unit from 4 to 10 LTO4 drives which meant that we had to add two new drives to the existing enclosure and then bolt on a 9u expansion unit to house the other 4 drives.
All of this went well and after running all of the library tests and coming up good we were ready to re-present this library back to the backup servers to be re-incorporated to our backup policies.
It was here that we ran into a problem which I suspect is quite common in this situation.
When we brought our backup server online again the server could now see 10 LTO4 drives, but could no longer see the 'unknown media changer' which appears within device manager representing the library configuration.
After being sent on a bit of a goose chase by a Quantum article stating that this is caused by the wrong HBA driver installed on our fibre HBA (The article states that this is the case when a storport driver is used instead of a scsiport or fcport driver and to check with the HBA vendor for the correct driver to resolve the issue) it turns out that the issue was a lot simpler to fix (isn't it always the case?).
On the library configuration portal there is an option within the 'Setup' menu called 'Control Path'. This had nothing configured when we checked our library after the upgrade and as this sets up one of the tape drives to present all of the library configuration through to the connected server this was obviously our issue.
To resolve this we simply selected one of the drives to be the control path and applied the setting. A quick re-scan of the devices within the media server and up popped our library and all of the correct slots etc.
The reason why this setting was no longer there was worrying to me though so I did some digging and it turns out that the way that the drive numbers are allocated can change when you add additional drives to the library, even if the original drives are not moved from their current slots.
Also, keep in mind that if you have more than one partition on your library that you will need to set the control path for each partition within the library.
PS: once you get the media changer presented to your server be sure to install the correct driver for the changer as this will then name the media changer with the make/model of the library etc.