Question: Seagate portable hard drive
Not so much a question as an answer, having got the seagate above, connected and not a lot happened, after messing around, set up in time machine which apparently knew the magic combination. Went back to seagate instructions, application>utilities>disk utilities. Partition at top open and choose Guip whatever, then click on Seagate go through erase and naming of disk as Seagate instructions. You should be good to go.
Reset Time machine for the external disc, stop i-cloud and i-drive if you wish.
iMac Pro, High Sierra