Time Machine: Excluded partition missing, HHD inaccessible

I installed a Seagate HHD, configured as Mac OS Extended, created 2 partitions, and encrypted using Time Machine.

It backed up the first day, then I decided to exclude, probably mistakenly, one of the partitions from backup. Now, only the other partition shows up and works, but the excluded one can’t be found via TM ‘options’, though the HHD can be verified on Disk Utility (but greyed out).

Moreover, my password to access the HHD is correct but but won’t provide access.

As well, in TM, it says it will do next backup: After Disk is connected…though the HDD is connected and regularly makes its noises.

I presumably made a mistake in excluding one of the partitions from TM backup…rebooting didn’t work. How can I get back on track?

Thanks for any insights. NF