Failed to detach disks during HotAdd Backup Operations

Products

• VMware vSAN

Issue/Introduction

During a scheduled backup using Backup Software, the following issues were observed:

• The backup job failed with a "Failed to detach disks from VM" alert.
• The system attempted an automatic "clean" action but could not find the corresponding disks for unmounting.
• Target disks on the Virtual Machine appeared in an "Offline" state.

Environment

VMware vSAN 8.x

Cause

The issue occurs within the HotAdd transport mode workflow. In this mode, the backup proxy "HotAdds" (mounts) the target VM’s virtual disks (.vmdk) to itself to read data and is supposed to "Detach" them once the backup is complete.

Similar logs in vmware.log:
YYYY-MM-DDTHH:MM:SS In(05) vmx - HotAdd: Adding scsi-hardDisk with mode 'persistent' to scsi1:1
YYYY-MM-DDTHH:MM:SS In(05) vmx - HotAdd: Adding scsi-hardDisk with mode 'persistent' to scsi1:2
YYYY-MM-DDTHH:MM:SS In(05) vmx - HotAdd: Adding scsi-hardDisk with mode 'persistent' to scsi1:3
YYYY-MM-DDTHH:MM:SS In(05) vmx - HotAdd: Adding scsi-hardDisk with mode 'persistent' to scsi1:0

YYYY-MM-DDTHH:MM:SS In(05) vcpu-0 - Destroying virtual dev for scsi1:0 vscsi=############
YYYY-MM-DDTHH:MM:SS In(05) vcpu-0 - Destroying virtual dev for scsi1:1 vscsi=############
YYYY-MM-DDTHH:MM:SS In(05) vcpu-0 - Destroying virtual dev for scsi1:2 vscsi=############
YYYY-MM-DDTHH:MM:SS In(05) vcpu-0 - Destroying virtual dev for scsi1:3 vscsi=############

Resolution

1. To check the Backup Proxy VM edit settings. If any disks belonging to the source VM are still attached to the Proxy, manually remove them .
2. In the guest OS of the Backup Proxy or the source VM, perform a storage rescan to clear the "Offline" status of the disks.