Hyper-V VM failed to start - Synthetic SCSI Controller (Instance ID ...) The system cannot find the file specified

Host server running Windows Server 2012 R2 Standard.
To VMs running.  VM1 is a domain controller and file server.  VM2 is Remote Desktop Services.
Things have been working fine for 6 months.
Yesterday, I added two external USB hard drives for use with Windows Server Backup in VM1 (to backup files on VM1).  I physically plugged them into the Host and then went through the process of making them offline so I could add them as hard drives in VM1.  That process went fine.  They showed up as DISK 2 and DISK 3 in VM1's Control Panel > Admin Tools > Computer Mgmt > Disk Mgmt.  I was able to configure the backup job via Windows Server Backup.  I selected the option to dedicate the disks to backup, so it reformats the external drives.  By design, the drives no longer show up in VM1's Windows Explorer, but they still show up in VM1's Disk Management.  Since I'll be rotating the drives for disaster recovery purposes, I unplugged one of the external drives and left one of them plugged in.  
This morning I went to check if the backup job ran.  It took several tries to get the Windows Server Backup snap-in to open.  I noticed the job had failed.  I decided to shutdown the VMs and also reboot the host server.  Upon reboot, I was able to login to the host, but when I tried to start VM1, it failed with a couple of error messages.  VM2 starts fine, but I really need VM1 as it is the domain controller and file server.  The errors all seem to have to do with Synthetic SCSI Controller and not being able to find file specified.  There's also an error that says something about failed to finish reserving resources because the system cannot find the file specified.
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Philip ElderTechnical Architect - HA/Compute/StorageCommented:
You need to REMOVE the drive in the VM's settings before unplugging the external drive.

Once REMOVED, swap the external drive, set to OFFLINE, then ATTACH to the VM.

Better to use a third party and a shared folder on the external drive for backups. This is how we do it for our standalone and small clustered virtualization solutions.

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