Hi Aussie_Joe,
Thanks for visiting the Apple Support Communities. Based on your screenshot, it appears as though Time Machine may be recognizing two separate Backup Disks: one that contains backups from as recently as February 21, 2023, while the other appears to be a new backup disk preparing its first backup. It also appears the new backup disk does not have any available space with which to create the backup.
To ensure no data is lost while we investigate if the above scenario is the case, we would recommend not deleting anything at the moment. Instead, start by verifying the correct Time Machine Backup Disk is connected to your Mac and all cables are secure and network connections active. Then, restart your Mac and check to see if Time Machine still lists both of these drives. This will help us start to isolate what is happening: Shut down or restart your Mac - Apple Support
Best!