Partition Type : FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF
The issue was fixed based on this answer: OS volume shows as type 'FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF'

It's unclear why the partition type was changed. On the original boot drive (disk0) Yosemite is installed. On the second "broken" drive (diisk1) an APFS partition was detected with the sudo dd if=/dev/disk1s2 count=3 | hexdump method from the linked answer.

After removing the second partition on drive1 with sudo gpt remove -i 2 disk1 the MBR blocked adding the partition with the proper type. So the gpt had to be destroyed:

sudo gpt -r show disk1 #get the details
sudo gpt destroy disk1
sudo gpt create -f disk1 #create a new partition table


Adding all previous partitions with the proper types:

sudo gpt add -i 1 -b 40 -s 409600 -t C12A7328-F81F-11D2-BA4B-00A0C93EC93B disk1
sudo gpt add -i 2 -b 409640 -s 820115416 -t 7C3457EF-0000-11AA-AA11-00306543ECAC disk1
sudo gpt add -i 3 -b 820525056 -s 156248064 -t EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 disk1


Verify the disk:

diskutil verifyDisk disk1


Since the version of the boot system is 10.10.5 (Yosemite) which can't deal with APFS volumes, it wasn't possible to verify the volume.



Further similar questions (and solutions) on this site suggest that the problem is related to the SIP protected MBR. Yosemite doesn't recognize SIP protected items but newer systems do. Here it was possible to kill the MBR, but on newer systems SIP has to be disabled.