Hi talkinghead250,
It looks like you're unable to remove a disk you are currently using for your Time Machine backups, even after you select "Stop using this disk". We know how important it is to have the correct storage device for those backups. You've come to the right spot for help with this.
Let's start with making sure your Mac is up to date with the most recent macOS your device is eligible to install:
How to update the software on your Mac
Once your software is up to date we'll want to try and remove the disk again.
Let us know if your Mac is in the process of creating a Time Machine backup or restoring data from a previous backup if you still are not able to remove the disk after installing any software updates.
Take care!