My external drive will not mount after installing 10.15.7 supplemental update. Was working prior to install.

Try connecting without the hub. You may still need a USB adaptor, but the hub is more complex (has more ways to fail) than a simpler adaptor.

If you can, try connecting the drive to another computer, preferably one with a direct USB connection where no adaptor is required.

Does the Seagate external drive have Seagate's custom software/firmware implemented? If it does, that might be a possible incompatibility, as the OS advances, sometimes these manufacturers' custom firmware is not compatible.

If none of the above provides any relief, try booting into Safe Mode, can you see the external drive then?

If still no success, I would take the drive to an Apple Authorized Service Provider. Easy to replace a backup drive but since this has Photos etc., you need to explore all options. The drive may have failed, my wife's external backup drive just failed last week. We replaced it with another backup drive and everything is backed up again, but unless your other files (photos ...) are also backed up, you will want to try a service shop if nothing else works.

My approach to backups is to keep two backups of all files that are important, in addition to the originals.