I have two external hard drives, WD-RED 2.0 TB, each is installed in a "Plugable" USB3-SATA-USASP1 SCSI Disk drive plugged into separate USB drives on my Dell Inspiron. (Drives F: and G The first is a tad full and will no longer fit another system image. So, I wanted to backup to the second which has plenty of space, being mostly unused. I just finished a file backup, to G: but cannot write a system image because Windows 10 does not show the drive as an eligible backup device!

The only difference I can see in properties of the two file systems is that the second one allows files and content to be indexed.