Re: Where are the backups?
Then they have misunderstood the principle of backup, which is supposed to be immutable!
That's an awful lot of BD-R disks.
Or do you mean offline rather than immutable? Tape storage is not immutable, but tends to be offline unless the tape has been loaded into a tape-drive and mounted*. The problem is ensuring that files being backed up to tape have not already been encrypted with a key you don't know (You are encrypting backups, aren't you?). There is nasty malware out there that encrypts your backups with the malware key for a while before triggering on the source data, so you find your backups are useless. Which is why a regular check to see that you can restore from backup on an independent system is important.
For me, the principles of backup include:
1) Backup data to backup media. Twice.
2) Disconnect backup media from system being backed up.
3) Attach backup media to independent system and check you can restore successfully.
4) Move successfully checked backup media to independent off-site locations.
5) Repeat frequently enough that restoral of backed up data doesn't give you a system with unusably old data on it.
Copying data from one network-attached system to another, leaving them permanently network-attached does not give you a secure backup. Verifying an offline copy of the backed-up data on an independent system is important.
*Note that automated tape silos are vulnerable to shenanigans, as are automated shingled-disk silos. Erasing one medium of the set from the previous couple of cycles of full-backups will kill the utility of the backup. Offline means - unable to access by means of programmatic control, and you need to be very sure that your backup-media inventory control system can't be compromised to 'accidentally' request operators to mount (for erasure) critical backup media.