Problem with back-up – Two of the disks to back up have the same name. Rename one of the disks named “OS X Base System”

Hi guys, I'm having some problems backing-up my Mac.

When I connect my external hard-drive and try to back-up I get the following message:

"Time Machine couldn’t complete the backup to “Elements”."

Two of the disks to back up have the same name. Rename one of the disks named “OS X Base System”.

I tried to rename one of the disks, but the other gets automatically renamed too. I also accidentally deleted my previous back-ups, so there's no way I can restore one of the previous ones.

Can you help me figure this out?

Thanks in advance.