🚀 NAS – P8 How to Migrate Proxmox VM Using Disk Passthrough to a New Host

Complete & Safe Migration Guide

Migrating a Proxmox virtual machine that uses disk passthrough requires special attention.

Unlike standard VM migration, passthrough disks cannot be moved automatically between hosts. This is where many users make mistakes that result in:

• ❌ Missing disks after restore

• ❌ DSM volume not detected

• ❌ Broken RAID configuration

• ❌ Data loss

In this guide, you’ll learn how to migrate a Proxmox VM to a new host safely and correctly.

This method is especially important for:

• 🗄️ NAS systems

• 🧩 Synology DSM virtual machines

• 🏠 Advanced homelab setups

• 🏢 Small production environments

We will cover how to:

• 🔍 Verify disk passthrough information

• 🔌 Detach disks properly

• 💾 Backup and restore the VM

• 🔗 Reattach passthrough disks correctly

Follow this step-by-step process to ensure a smooth and reliable VM migration.

📌 Why Disk Passthrough Migration Is Different

In normal Proxmox VM migration:

• Virtual disks (qcow2/raw) move with the backup

• Storage abstraction handles disk relocation automatically

With disk passthrough:

• Physical disks remain on hardware

• They are NOT included in the VM backup

• They must be manually reattached on the new host

⚠️ This is why careful verification is critical.

🛑 Step 1 – Shut Down the VM

Before starting migration:

Properly shut down the virtual machine.

This prevents:

• File system corruption

• RAID inconsistency

• DSM database issues

Never migrate a passthrough-based NAS VM while running.

🔍 Step 2 – Get Disk Passthrough Information

Before detaching anything, record the exact disk ID.

Run:

Example output:

Disk passthrough: Samsung

📌 Important:

Always use /dev/disk/by-id/ instead of /dev/sdX.

This ensures consistent disk identification after reboot or hardware change.

Document this disk ID carefully — you will need it later.

🔌 Step 3 – Detach Disk Passthrough

Detach the passthrough disk from the VM configuration.

This ensures:

• The VM backup does not reference unavailable hardware

• The configuration remains clean

• The restore process works properly

After detaching, confirm that the VM no longer lists the passthrough disk.

💾 Step 4 – Backup VM NAS

Now perform a full VM backup.

Back up the VM to another storage location such as:

• SMB

• USB box

• OneDrive

• NFS

The passthrough disk will NOT be included in this backup.

You are backing up:

• Boot disk

• VM configuration

• DSM system partition

This backup will later be restored on the new host.

♻️ Step 5 – Restore VM on the New Host

On the new Proxmox server:

Restore the VM from backup.

This demo assumes a vma.zst backup already exists; proceed with the restore.

After restore:

• The VM will exist

• Boot disk will be present

• Passthrough disk will NOT yet be attached

Do NOT start the VM yet.

🔗 Step 6 – Add Disk Passthrough Back and Start the VM

Now reattach the physical disk using the previously recorded disk ID.

Run:

Explanation:

• 103 → VM ID on new host

• scsi0 → First data disk

Make sure:

✔ The disk exists physically on the new host
✔ The disk ID matches exactly
✔ Boot disk remains correctly configured

Now start the VM.

DSM should detect the disk and recognize the existing volume.

🌐 Step 7 – Add the NFS Shared Storage Again

If your VM previously used NFS shared storage:

Reconfigure the NFS storage on the new host.

This ensures:

• Backup paths remain valid

• ISO storage reconnects

• Shared storage environment remains consistent

Once completed, your NAS environment should be fully operational again.

🛡️ Common Mistakes to Avoid

❌ Forgetting to record disk ID
❌ Using /dev/sdX instead of by-id
❌ Starting VM before reattaching disk
❌ Attempting live migration with passthrough disk
❌ Forgetting to reconnect shared storage

Disk passthrough migration is simple — but only if done carefully.

🏢 Real-World Scenario

Example case:

• Old Proxmox host hardware upgrade

• New server installed

• Physical NAS disks moved to new server

• VM restored from vma.zst backup

• Disk reattached using by-id

Result:

✔ DSM boots normally
✔ RAID volume recognized
✔ No data loss
✔ Minimal downtime

This method is widely used in:

• Hardware upgrades

• Host replacement

• Infrastructure restructuring

🎯 Final Thoughts

Migrating a Proxmox VM that uses disk passthrough is not the same as migrating a standard virtual machine.

Because passthrough disks are physical devices, they require:

• Manual verification

• Proper detachment

• Correct reattachment

When performed correctly, this process ensures:

• Data integrity

• RAID consistency

• Stable NAS operation

• Smooth hardware transition

For NAS administrators and virtualization engineers, mastering passthrough migration is an essential skill.