Can't erase my external hard drive (problems with hard drive after updating OS to Catalina)
I have a 5TB external drive, I updated my OS to Catalina earlier this week and when I put the ext drive back in it said the disk "couldn't be repaired" and that I needed to reformat it. After many hours backing up the 3.5TB of data I had on there, I finally went to Disk Utility and selected to erase the entire disk, I reformatted to MS-DOS (although I think this is what it was originally). This completed fine, according to the dialog box, but after clicking 'Done' it seems like the disk was partitioned, it now has 4.4TB used and I can't get it to erase this data, no matter how many times I ask it to erase it stays there, I went into the partition tab, but it won't let me remove the partitions - the plus/minus buttons. I just want a clean drive again and there doesn't seem to be any way of doing this.The drive wasn't even attached to the computer when I updated the OS so I don't understand how this has happened.
I've attached screenshots in case that's helpful to anyone, the drive I want to erase is called THE BRAIN.
MacBook Pro 15", macOS 10.15