Why am I being asked for a disk password on boot? I have multiple disks

I recently migrated to a new iMac.

Upon boot, I am getting a "disk password" prompt every time. I don't want this. I have three disks, one internal, and two external.

Internal disk: CoreStorage Logical Volume • Mac OS Extended (Journaled, Encrypted)

External disk 1 (time machine): CoreStorage Logical Volume • Mac OS Extended (Case-sensitive, Journaled, Encrypted)

External disk 2 (extra storage): APFS Volume • APFS (Case-sensitive, Encrypted)

I'm guessing it's external disk 2 that is the problem somehow. I'm not sure why OS X does not remember its password and prompts me every time. How do I undo this?

Thanks!