Thanks everyone for their help. I believe I have a way forward now.

Or could I get a one off backup of those VMs by disconnecting that disk temporarily?

Is there a possibility of converting that SR to thin provisioning - would that help in this case?

@Danp Thanks Dan - that makes sense - however one of those 6 x VMs is backing up and snapshotting perfectly, and it has the extra disk too..
Why would this be?

The SR of the host is 51% full.

The VM:

The error log for the snapshot failure:

vm.snapshot
{
  "id": "567459b9-362c-90e8-9b2b-9e61572243ff"
}
{
  "code": "SR_BACKEND_FAILURE_44",
  "params": [
    "",
    "There is insufficient space",
    ""
  ],
  "task": {
    "uuid": "f2c46023-ea35-7d61-1106-b799adec4648",
    "name_label": "Async.VM.snapshot",
    "name_description": "",
    "allowed_operations": [],
    "current_operations": {},
    "created": "20250829T10:31:09Z",
    "finished": "20250829T10:31:16Z",
    "status": "failure",
    "resident_on": "OpaqueRef:b71238e8-bce1-4a59-b9be-870e2de57558",
    "progress": 1,
    "type": "<none/>",
    "result": "",
    "error_info": [
      "SR_BACKEND_FAILURE_44",
      "",
      "There is insufficient space",
      ""
    ],
    "other_config": {},
    "subtask_of": "OpaqueRef:NULL",
    "subtasks": [
      "OpaqueRef:09be2d2a-c450-42fc-8c3a-3c876274bb18"
    ],
    "backtrace": "(((process xapi)(filename ocaml/xapi/xapi_vm_clone.ml)(line 80))((process xapi)(filename list.ml)(line 110))((process xapi)(filename ocaml/xapi/xapi_vm_clone.ml)(line 122))((process xapi)(filename lib/xapi-stdext-pervasives/pervasiveext.ml)(line 24))((process xapi)(filename lib/xapi-stdext-pervasives/pervasiveext.ml)(line 35))((process xapi)(filename ocaml/xapi/xapi_vm_clone.ml)(line 130))((process xapi)(filename ocaml/xapi/xapi_vm_clone.ml)(line 171))((process xapi)(filename ocaml/xapi/xapi_vm_clone.ml)(line 209))((process xapi)(filename ocaml/xapi/xapi_vm_clone.ml)(line 220))((process xapi)(filename list.ml)(line 121))((process xapi)(filename ocaml/xapi/xapi_vm_clone.ml)(line 222))((process xapi)(filename ocaml/xapi/xapi_vm_clone.ml)(line 442))((process xapi)(filename lib/xapi-stdext-pervasives/pervasiveext.ml)(line 24))((process xapi)(filename lib/xapi-stdext-pervasives/pervasiveext.ml)(line 35))((process xapi)(filename ocaml/xapi/xapi_vm_snapshot.ml)(line 33))((process xapi)(filename ocaml/xapi/message_forwarding.ml)(line 131))((process xapi)(filename lib/xapi-stdext-pervasives/pervasiveext.ml)(line 24))((process xapi)(filename ocaml/xapi/rbac.ml)(line 205))((process xapi)(filename ocaml/xapi/server_helpers.ml)(line 95)))"
  },
  "message": "SR_BACKEND_FAILURE_44(, There is insufficient space, )",
  "name": "XapiError",
  "stack": "XapiError: SR_BACKEND_FAILURE_44(, There is insufficient space, )
    at Function.wrap (file:///opt/xo/xo-builds/xen-orchestra-202408190902/packages/xen-api/_XapiError.mjs:16:12)
    at default (file:///opt/xo/xo-builds/xen-orchestra-202408190902/packages/xen-api/_getTaskResult.mjs:13:29)
    at Xapi._addRecordToCache (file:///opt/xo/xo-builds/xen-orchestra-202408190902/packages/xen-api/index.mjs:1041:24)
    at file:///opt/xo/xo-builds/xen-orchestra-202408190902/packages/xen-api/index.mjs:1075:14
    at Array.forEach (<anonymous>)
    at Xapi._processEvents (file:///opt/xo/xo-builds/xen-orchestra-202408190902/packages/xen-api/index.mjs:1065:12)
    at Xapi._watchEvents (file:///opt/xo/xo-builds/xen-orchestra-202408190902/packages/xen-api/index.mjs:1238:14)"
}

Just too add something too. The 1 x VM that snapshots fine had management agent 9.4.1 on it - and the other 5 have 9.3.3. So I assumed this might have been the issue - and upgraded one of them to 9.4.1 as a test. Still same error.

Note that about 10 other VMs on the host snapshot and backup just fine.

@Danp Hi Dan, there is only one Unhealthy VDI, but about 110 (500mb) Orphan VDIs....?

Most backups working fine. 5 not working. End with "SR_Backend_Failure_44 (There is unsufficient space)". Those 5 x VMs will not snapshot either. The host is 8.2.1.

Local storage on the host is 51% used.

This is one of the VMs that won;t backup, however there are 6 identical, and only one of them backs up correctly.

Any ideas?