A Proxmox VM getting locked is commonly caused after a VM backup is interrupted or crashed. Simply unlocking the VM through SSH using the following command will fix this issue:

# qm unlock <vm_id>

By default, Proxmox backup will back up all the virtual disks assigned to a VM. If we want to exclude certain virtual disks from the backup process, we only need to add the option backup=no at the end of a virtual disk line item in <vm_id>.conf, as follows.

virtio0: rbd-hdd-01:vm-101-disk1,size=80G
virtio0: rbd-hdd-01:vm-101-disk2,size=200G,backup=no

In the previous example, the virtual machine has two virtual disks. Disk 1 is for the primary OS and disk 2 is for the secondary. By adding backup=no, Proxmox will skip this disk during the backup process and only back up the primary disk.