On a system with old-school MSDOS partition tables and a kernel RAID'ed / or /boot, grub isn't installed to md0, it's installed directly to each member drive, and each member drive is individually ...

The system will work fine if you have both boot drives as MBR and the 4TB drive as GPT. (Maybe even get another small SSD to use as the backup OS instead.

The disk to which the backup was being restored must have been using MBR partition table before. Doing so causes the GPT partition table to overwrite the data on the destination drive.