Problems reformatting new ext HDD using High Sierra

Hello! I'm unable to erase and format my external HDD to mac OS ext using my new iMac and High Sierra. It gives me an error and afterwards, makes the drive unreadable. I am able to erase and format the drive correctly on my old iMac using Snow Leopard, which is weird. The drive then also works on High Sierra. It identifies 2 partitions preexisting on the drive before reformatting. Is this part of the problem?

The external HDD is a seagate backup plus 5 TB.

I'm able to format using my old computer for now but when my old Mac dies I'll need a better solution.

Thanks for your help!