I followed the same single drive guide to install Ubuntu 24.04 as referenced in the linked (below) other thread here. Unlike the other thread author, I made an EFI partition on my second drive and have a zfs mirror of the other two similar sized partitions. I used dd to clone the EFI partition, but the 2nd EFI partition isn’t bootable. What do I need to do to rectify this situation?
you should be able to fix that by creating an EFI boot entry for it using efibootmgr. Just use the command from the original installation that created the entry for the EFI image on the first drive, but change the drive id.