External SSD drive ignored in timemachine options

New iMac 27" running Big Sur.

Set up. 2TB internal SSD, 4TB Oyen Digital U32 Shadow external SSD (working fine for data and about 60% full), 2 TB (empty, for now) additional HDD. Backing up to 6TB Seagate HDD APFS formatted (2 volumes 4 and 2 TB).

The first timemachine backup has just completed correctly as far as I can see for the internal drive. However, it ignored the external SSD drive. I had excluded in options the smaller HDD.

In SySPref > Timemachine > Options the SSD drive is on the list but as a faded/dull list item compared to the other drives. I can exclude or include the smaller hard drive that appears bright when on the list. In the screenshot below I had included it again to show the only dull listings are the backup drive and the U32. I cannot change anything for the U32 Shadow.

Ideas please.