If you have a Backup Disc from a Proxmox Node and if you want to use this in a new Node or Server, you have to mount manualy the disc.
attach the usb drive to server
-> pve -> discs Now you see your usb drive, for example sda1
open the shell
we accept the mount name is USB
mkdir -p /mnt/pve/USB
chmod 777 /mnt/pve/USB
nano /etc/fstab
here we need to add the line
/dev/sda1 mnt/pve/USB ext4 defaults 0 2
2 is the incremental number
sda1 ist USB Drive seen in pve – discs
we save all and exit from nano
mount -a
this mount now the drive, and in the backup section you will see your backup files
My prefered way is to migrate over a cluster with a second node, then you need only to migrate every vm to the new node.