Hyper-V 2016 Known Issue with Removable Disk
In Hyper-V 2016 Standard and Rapid Recovery, Resilient Change Tracking (RCT) returns an error when a removable disk is removed during backup operations. Error Encountered: “Unable to create a reference point for virtual machine “VM Name”
A reference point for the Virtual Machine (VM) could not be created. Reference points are needed to be able to take an incremental backup of the Virtual Machine. Failure to create a reference point may occur if any of the storage devices used by the Virtual Machine are not accessible at the time of the backup.
Resilient Change Tracking (RCT) was introduced in Hyper-V 2016 and it provides the baseline for change tracking in Hyper-V VM's instead of relying on the CBT driver provided by Intronis Backup.
This is a Hyper-V 2016 issue that Microsoft is planning to address in a future update.
Ensure the storage device is attached on the affected virtual machine, export and then delete the virtual machine, re-import the exported virtual machine, and retry the backup.