When using vRanger to backup Hyper-V VM's, Event Viewer warning with the 157 ID is present in guest's system log.
This Event occurs after a backup task completes and it refers to a disk that is currently not connected to the VM.
vRanger as a VSS Requestor depends on the functionality of VSS providers present in the Windows OS.
VSS Reports the Event Viewer warning because of the Snapshot process that takes place.
Snapshot is mounted as a VHD file which at the end of the backup is removed, resulting in "Disk # has been surprise removed" System Event log.
This warning can be safely ignored.
This article details more information about the Event ID 157 "Disk # has been surprise removed"