How to Use Extra Space on an APFS Time Machine Drive
When buying a drive for Time Machine backups, it’s always a good idea to get one that’s larger than you expect you’ll need—plan for the future! If you need to offload gigabytes of non-backup files from your internal drive, either temporarily or as a permanent archive, it’s tempting to take advantage of that 4 TB sitting unused on your 8 TB Time Machine drive. By default, you can’t use the free space on an APFS Time Machine drive for anything else. However, Apple says you can if you create an additional APFS volume on the disk. In an Eclectic Light Company post, Howard Oakley refines Apple’s advice to explain when you should create a new volume within the Time Machine APFS container and when you should instead create a separate APFS container. Remember that you still need to back up those extra files on the Time Machine drive in some other way!
This is also great advice for all those who wish to benchmark their TM drives. Normally your benchmarking tools cannot write to the APFS TM volume, but as soon as you add another volume to the container, you’re good to go. And when you’re done benchmarking, you are of course free to hand that space back to the TM volume if you wish by simply deleting it the same way it was added.