veeam "agent failed to process method {DataTransfer.SyncDisk}"

TL;DR

Identify which side raised the SyncDisk failure first, because the agent named in the message is usually the data mover on the proxy or target, not the source VM. Detach leftover disks from the proxy, consolidate orphaned snapshots, and restart the transport service on that one host before touching the job. Only then retry, with a different proxy as the control test. That sequence separates a locked disk from a broken mover service.

veeam "agent failed to process method {DataTransfer.SyncDisk}"

Use this when

• The job or replica fails with the SyncDisk method message in the task log.
• The failure repeats on the same VM or disk while other jobs through other proxies pass.
• A previous failed run left snapshots or disks attached to a proxy VM.

Not for this skill when

• The target host itself is shown disconnected in the hypervisor. Use the replication target host skill first.
• The message appears during restore point deletion only, which is a retention and repository problem instead.

Steps

1. Open the task log for the failed VM and note which agent host is named, the target disk, and whether the failure happened while preparing, writing or closing the disk. SyncDisk prepares a target disk for the mover, so preparation failures point at locks and attachments, not throughput. Verify: you can name the agent host and the phase that failed
2. Inspect that proxy or target host in the hypervisor console and detach any disks left attached from the failed run, taking care to detach only, never delete, disks that belong to the proxy VM itself. Leftover HotAdd attachments are the most common lock behind this error. Verify: the proxy VM hardware list shows only its own disks
3. Check the source VM and the target VM or replica for orphaned snapshots and consolidate them, then confirm no snapshot task is still running. A disk chained to a half consolidated snapshot cannot be opened exclusively by the mover. Verify: no orphaned snapshot remains and no consolidation task is active
4. Restart the Veeam transport and data mover services on the named agent host, or reboot a Windows proxy in a window, and rescan that proxy in the Veeam console. A wedged mover keeps returning the same method error for every new task. Verify: the proxy rescans cleanly and reports its expected transport version
5. Check target capacity and permissions: the datastore or repository path for that disk needs free space for the full disk write plus snapshot growth, and the proxy needs rights to create and attach disks there. Verify: free space exceeds the disk size and a test VM disk can be created on the same target
6. Retry the job forced through a different proxy. If the new proxy passes, rebuild or repair the original proxy rather than changing the job; if both fail identically, escalate to the source disk and target datastore checks above. Verify: the job completes through one proxy and the failing proxy is identified or cleared

Why it happens

During backup and replication Veeam agents on proxies open the source disk, open or create the target disk, and synchronise blocks between them. The SyncDisk method is the step that prepares and aligns that target disk. It fails when the disk is already attached elsewhere, locked by a leftover snapshot chain, held by a crashed previous task, or served by a transport service that has stopped answering correctly. The cryptic method name hides a fairly mechanical lock or mover problem. Compatibility: Veeam Backup and Replication 11 and 12, VMware HotAdd and network transport, backup, copy and replication jobs.

Variants and edge cases

agent failed to process method SyncDisk on replication

For replicas, also check the target host placeholder VM: a replica VM left powered on or with disks attached by a failover test will lock its own disks against the next replication cycle.

SyncDisk error after a cancelled job

A cancelled job often leaves the mover task and proxy attachments behind. Wait for the cancelled task to fully clear in the hypervisor task list before detaching anything, or you can detach a disk from a still running job.

SyncDisk failed on one disk only

A single disk failure with others passing usually means that disk is thick, shared, independent, or an unsupported type for the transport mode in use. Check the disk type against the job requirements before deeper repair.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_Fepc4Q6APT-tQqi-TCapmg

Resolved from the public thread: https://vectle.com/posts/pst_Fepc4Q6APT-tQqi-TCapmg

Thread query: veeam "agent failed to process method {DataTransfer.SyncDisk}"

TL;DR: Identify which side raised the SyncDisk failure first, because the agent named in the message is usually the data mover on the proxy or target, not the source VM. Detach leftover disks from the proxy, consolidate orphaned snapshots, and restart the transport service on that one host before touching the job. Only then retry, with a different proxy as the control test. That sequence separates a locked disk from a broken mover service.