as said by
fortechitsolutions, you need to either combine all those extra disks into a single array in your vm host, once in an array, probably your easiest route is to format the array with some file system ie:
Code:
mkfs.ext4 /dev/md0 #(assuming you made a mdadm array).
mkdir /mnt/backups
mount /dev/md0 /mnt/backups
Next in proxmox web gui add that storage as a directory: /mnt/backups
On the add storage dialog there is a control called Max Backups, calculate your total guest storage usage vs available space on array, ie if arrays is 2000gb, and 4 guests have used space of 90gb each, you should limit your backups to 4 to prevent filling drive... make sure to leave space for extra backup, as I think proxmox deletes the oldest backup only after a new one is created.
The above is the path of least resistance and easiest to grasp with minimal knowledge of file systems. You could also use LVM, or zfs, but you need to really understand how they work or you could break something bad.
Using zfs/lvm- it must have a formatted file system to store backups (snapshots should not be considered backups). Only running VMs can sit on raw zfs/lvm.... once formatted (ie ext4), you would mount it to a dir (/mnt/backups) add it in the pve web gui as a
directory not as a zfs pool (only for running vms).
Or - if your VMs are already running on zfs, then you actually could use a 2nd storage running raw zfs without a file system and use pve-zsync to sync the file system over to the other drives, at the moment this would be completely managed from the command line, no gui.
https://pve.proxmox.com/wiki/PVE-zsync From your comments, you do not seem to understand the different layers of zfs, I suggest you read a lot more on it before trying to use it.