Fail to erase a backup disk.

Hi guys! I copied the files from a previous backup disk "Seagate Backup" to a bigger one and now Time Machine shows the new external drive as the backup disk. When I tried to erase the previous backup disk to use it I've got the message: "The volume “Seagate Backup ” on disk4s2 couldn’t be unmounted because it is in use by process 527 (mds_stores). Couldn’t unmount disk. : (-69888)"

How can I format the old backup disk to use it as an expansion of my internal hard drive?

I use the Mac Studio with M1 chip.

Thanks a lot for any help!