Time machine jumps on other hard drive when backup drive is removed

Ok, so this problem is annoying me to no end. I have two usb powered hard disks, one from Western Digital, and the other from Verbatim. The WD disk was used as a project drive and for my time machine backups. I quickly realised that TM likes to use all available space so I got a the Verbatim one, figuring that I should have one drive exclusively for my backups.

The backup was transferred to my new disk and it was assigned the new TM disk. It has no problems backing up to it, but something happens consistency when unplugging the TM drive.

After sleep and upon waking I am greeted with this message.

I click "Don't Back Up", but opening TM preference I see this (notice the "oldest" and "latest backup" date/time). It's also decided it wants to be a TM disk.

When I connect my REAL TM Disk it just jumps in like nothing has happened. Ready to continue backing up (again, look at the date/time. Identical as my project disk).

There seems to be some confusion of identity (or some hidden files). But they don't share the same name, the backup folder on my project disk is deleted. So what's happening?