It is my pleasure to inform you that your support person Jia effectively helped us solve the problem for our client.
He guided us to troubleshoot by trying the HP Library and Tape Tools. In the software, only one drive was shown (unlike Device Manager where 2 drives were shown). Therefore, we knew it wasn't Veeam's fault.
It was found that on our SAN switch, one of the two aliases of the tape drives had an invalid value (member) -- it was changed to an unknown value for an unknown reason, while the other alias was OK; therefore, only one drive was detected. After correcting the bad alias member (and removing the offline libraries, media pools from Veeam, restarting all Veeam services) and the two drives appeared.
Had the case not been on hold by our client, we would have solved it sooner.
Thanks a lot.