Can’t Resize Partition on External Drive

El Cap 10.11.6

Seagate Backup + external

The external drive has two partitions: one for Time Machine backups, and one for Files.

I want to increase the size of the Backup partition.

It will not let me drag the slider to enlarge it. It will let me reduce, but it creates another small partition (basically splitting it).

If I try to reduce the Files volume, it creates an additional partition. (same as above)

If I let it add the additional 500gb part., then delete it, the space is given back to the Files volume. Like, I can't even combine the new 500 with the old Backup.

Yes, I've tried to manually key in a larger size for Backup. It does not work.

Seems like a pretty straight forward task. Do I have something set wrong somewhere?

thanks in advance