Can't add third external disk for Time Machine

I'm trying to add a third external disk for time machine. the disk is Mac OS journaled and has been used with time machine before, but not with this Mac. Time Machine refuses to allow me to add the disk for backup although it shows up in options->exclude from backup.

I've removed one of the existing disks that's not currently connected. Didn't help. AFAICS I'm following all the process correctly but Time Machine refuses to add the disk to list of backup disks.

[Re-Titled by Moderator]