NetBackup sends the restore request to the same media server that did the backup and needs a drive on that media server with the same density.
If your 2nd library fits this criteria, just put the media required in the library, inventory the library, and you're good to go.
If the original media server cannot see the drives in 2nd library, you will have to assign the restore to a media server that does. You need to add FORCE_RESTORE_MEDIA_SERVER =
source_media_server new_media_server in master's bp.conf (or via Host Properties).
If the original backup was written on e.g. LTO 2 (hcart2) but the 2nd library has LTO 3 drives (hcart3), you will have to change at least one drive to hcart2.