System backup to another drive

[Tyrannocaster]

Using Timeshift v19.08

Timeshift - Settings - Location allows me to specify a USB connected drive as the destination of the system back up.  The SATA attached 'data' drive also appears as an option.

One of my ATA drives shows in that list but it tells me SELECTED DEVICE DOES NOT HAVE LINUX PARTITION. However, so does the system SSD which Linux is installed on. When I try to use the ATA drive, Timeshift goes to an apparent default of the supposedly unavailable SSD and backs up the snapshot there.

Literally everything I can click on in the Settings/Location window says I can't use it when I click on it. Plainly, at least one of those is wrong, but in any case I do not seem to be able to use the ATA drive for this.