One of the external backup disks I use to back up my personal laptop filled up when I was too busy to buy another, so (in a rush) I grabbed another project's backup disk and added my laptop to it too. Months later, now that project's backup disk is full, and I have finally bought a new backup disk for my own laptop. I need to free up the space again on the project's backup disk, for the project.
I tried formatting the new disk and was surprised to find that Apple now recommends using APFS to format time machine back ups. I did this, I used the new format in reformatting the disk. Now though when I try to drag my backup from the old backup disk, I get a pop up that says "The volume is the wrong format for a backup."
Is it not possible to move old laptops to an old disk? Do I just need to backup from scratch and delete these months of backups (I do have 2 other backup disks in other locations so that's not the biggest of deals, but it still rubs me the wrong way to lose data.) Or should this work? Would it work if I just reformat the new disk with the old (deprecated?) Apple "extended journaled" format?