If the backup jobs from the standalone drive reflect the random barcode, you could try right-clicking on the volume, and then "update barcode" to change it to the external barcode. Make sure you match up the internal and external labels properly. You may have to access the "assigned media" on the standalone library sub-tree to access any volumes not in the new library.
This is assuming you did not start up a new Commcell with the installation of the library. Otherwise you may need to use the catalog function referenced above.
Another thing - make sure the character length matches - ie, some libraries default to using six characters, and others use the "L7" on the end as an 8 character. Commvault considers those two different volumes.