Attaching One Proxmox VM Disk to Another VM

Attaching One Proxmox VM Disk to Another VM

This article explains how to attach an existing Proxmox VM disk to another VM as an additional disk, using both GUI (Web Interface) and CLI (command line) methods.

The guide applies to standalone Proxmox servers (no cluster, no shared storage) and focuses primarily on the default LVM-thin (local-lvm) storage, while also covering file-based storage (local, qcow2/raw).

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Important Concepts (Read First)

1. A disk can only be attached to one running VM at a time

• Attaching the same disk to two running VMs will corrupt the filesystem.

• You must power off VMs before detaching or attaching disks.

2. Disk types in Proxmox

Storage	Disk Type	Example Location
local-lvm	LVM-thin logical volume	/dev/pve/vm-100-disk-0
local	File-based (qcow2/raw)	/var/lib/vz/images/100/

The procedure depends on where the disk is stored.

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Scenario 1: Attach an LVM-thin Disk (local-lvm) to Another VM (Recommended)

This is the most common and safest scenario.

Example

• VM 100 → Source disk

• VM 200 → Destination VM

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GUI Method (Web Interface)

Step 1: Shut down both VMs

From the Proxmox Web UI:

• Select VM 100 → Shutdown

• Select VM 200 → Shutdown

⚠️ This step is mandatory.

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Step 2: Detach disk from VM 100

1. Select VM 100

2. Go to Hardware

3. Select the disk (e.g. scsi0)

4. Click Detach

5. Do NOT select Delete

The disk now exists but is unused.

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Step 3: Attach disk to VM 200

1. Select VM 200

2. Go to Hardware → Add → Existing Disk

3. Choose:

   • Storage: local-lvm

   • Disk: vm-100-disk-0

   • Bus/Device: VirtIO SCSI (recommended)

4. Click Add

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Step 4: Start VM 200 and mount the disk

Inside the guest OS:

lsblk

Example output:

sda    50G  (system)
sdb    100G (attached disk)

Mount the disk:

mkdir /mnt/attached_disk
mount /dev/sdb1 /mnt/attached_disk

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CLI Method (Faster & Scriptable)

Step 1: Shut down both VMs

qm shutdown 100
qm shutdown 200

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Step 2: Detach disk from VM 100

qm set 100 --delete scsi0

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Step 3: Attach disk to VM 200

qm set 200 --scsi1 local-lvm:vm-100-disk-0

You can verify:

qm config 200

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Step 4: Start VM 200

qm start 200

Mount the disk inside the guest OS as shown earlier.

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Scenario 2: Attach a File-Based Disk (local, qcow2/raw)

GUI Method

1. Shut down both VMs

2. Detach disk from source VM (do not delete)

3. Destination VM → Hardware → Add → Existing Disk

4. Select:

   • Storage: local

   • Disk: vm-100-disk-0.qcow2

5. Start the VM

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CLI Method

qm shutdown 100
qm shutdown 200
qm set 100 --delete scsi0
qm set 200 --scsi1 local:100/vm-100-disk-0.qcow2
qm start 200

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Scenario 3: Copy the Disk Instead of Moving It (Safer)

If you want VM 100 to keep its disk, create a copy.

Clone an LVM-thin disk

lvcreate -s -n vm-100-disk-0-copy /dev/pve/vm-100-disk-0
qm set 200 --scsi1 local-lvm:vm-100-disk-0-copy

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Copy a qcow2 disk

cp /var/lib/vz/images/100/vm-100-disk-0.qcow2 \
   /var/lib/vz/images/200/vm-200-disk-1.qcow2

qm set 200 --scsi1 local:200/vm-200-disk-1.qcow2

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Common Problems & Solutions

Problem	Cause	Fix
VM won’t boot	BIOS mismatch	Match SeaBIOS/UEFI
Disk not visible	Wrong bus type	Use VirtIO SCSI
Filesystem errors	Dirty shutdown	Run fsck
Windows disk missing	No VirtIO driver	Install VirtIO ISO

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Best Practices

✔ Always power off before attaching disks ✔ Use VirtIO SCSI for best performance ✔ Prefer cloning over moving for safety ✔ Never share a writable disk between running VMs ✔ Use NFS/SMB if you need shared data

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Summary

• Proxmox disks can be safely reattached between VMs

• local-lvm disks are easiest to manage

• GUI and CLI methods achieve the same result

• Disk copying is safer than disk moving

This method is commonly used for data recovery, VM consolidation, forensics, and migration workflows.

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End of article