Unable to select disks to verify

I have not seen this (bug). If you select a disk, only one disk should be selected. I don't know what you are doing, it sounds unlikely. When you click a disk tile, you are saying all the other disks are auto selected?

Notice that when I clicked on SATA bus 0, that the tree view had yellow and green parts. The green ones correspond to the internal drive, and the yellow ones correspond to my SSD (on USB 3 bus.) I currently have the other external USB drives disconnected; they were showing up as yellow before.

There is no separate USB2 disks tile, and clicking on the USB3 bus tile does not highlight any drives.

Ah, I notice that as I click on the individual Volumes, T5 does highlight to USB 3, and I can get individual connections for the partitions that are internal. The verify should be finished in not long; I will reconnect the external USB and see whether I can get individual highlights.

Now that I have reconnected my external USB, the behaviour has changed. Now clicking on SATA bus 0 only highlights the internal drive partitions, using only a green tree. Clicking on USB3 bus 0 ID 10 highlights the partitions on my external USB. Clicking on USB3 bus 0 ID 7 does not highlight any partitions. None of those tiles highlight the partitions on my external SSD. In the volume view, clicking on the non-APFS partitions highlights back to their respective disk on the left, including when I click on the HFS+ on the SSD (even though the HFS+ partition of the SSD was not highlighted when I clicked on the respective Disk tile.) Clicking on the APFS partitions and containers in the Volume view does not highlight anything.

I do appear to be able to verify a drive at a time now by clicking on the appropriate Disks tile. I do not appear to be able to verify a particular partition (an older post implies that I should be able to verify a particular partition.)

Verify is a disk function, not a volume function. You can "validate" a volume, not a disk, but only SoftRAID volumes.

SoftRAID does not support APFS in the user interface yet, so do not get confused by that.

I do not see what SoftRAID is seeing, but it appears you have a internal drive that is failing. If this is an iMac, you may be able to replace it. Else it is an Apple service thing, and they won't replace it until it has totally failed.

Verify of the internal drive cleared up the error flag.

I have a (different, not previously mentioned) external drive that is failing badly, and perhaps that is what triggered the flag on SATA Bus 0. The failure of that drive is what drove me to go for a RAID system; I am in the "try everything out before I trust it" stage.