Temporary Disks Fail to Attach During the Proxmox Backup

Product version: 11.0

Last modified: 17 October 2025

Problem

Proxmox backup job failed because one or more temporary virtual disks could not be attached to the Transporter.

Nbd Devices Insufficiency

This issue may occur due to one of the following reasons:

• There are fewer available nbd devices than source disks.

• The source Proxmox host was out of free nbd devices at the moment the job was running.

Solution

To solve this issue, increase the number of nbd devices on the Proxmox host source with the following command, where "X" is the number of nbd devices you want to be allocated to the host:

$ sudo modprobe nbd nbds_max=X

Note

After adding a Proxmox VE cluster or host to the inventory, the transporter creates 64 NBD devices on each host automatically.

Temporary disks couldn’t attach to the Transporter

The issue occurs on CIFS/SMB share datastores when restoring a VM that contains an LVM disk whose size is not a multiple of 4096 bytes. In this case, Proxmox cannot mount the RAW disk in write mode, causing the backup or restore to fail.

Solution

To solve this problem, resize the disk to be a multiple of 4096 bytes.