Thanks for getting back.
I'll try again overnight tonight, but if I get the same error, I'll have to look for a replacement for Macrium Reflect.
The error isn't related to Macrium Reflect, so replacing the software won't resolve the problem. It's a problem with the disk storing the backup file (the 12TB disk) and you'll likely have problems just copying large files to it using Windows Explorer. You may want to confirm this as the next troubleshooting step.