One external SSD for multiple Tahoe MacBook Time Machine backups

This isn’t a missing UI button, it’s how Time Machine now auto-claims an APFS target in Tahoe, when the second Mac sees an existing TM volume, it doesn’t create a separate backup namespace anymore, it just tries to reuse it, and that leads to snapshot contention and ownership conflicts, which is why the first Mac then starts failing.

The Apple-approved way hasn’t really changed under the hood, each Mac must have its own APFS volume or partition on that SSD, full stop. Don’t rely on TM to “figure it out” automatically. Open Disk Utility on one Mac, add a new APFS volume (or partition if you want hard size limits), then explicitly select that volume as the Time Machine disk on the second Mac. Once each Mac is writing to its own volume, the errors go away because each machine gets its own snapshot history, UUIDs, and space accounting, which is how Time Machine actually expects to work.

Two options, both involving Disk Utility.

1) Partition the drive. GUID partition scheme, AFPS format, non-encrypted (let TM handle that). Select sizes that are appropriate for the Macs, recommendation is 2-2.5x the internal drive, for for example if backing up Macs with 1 TB and 500 GB internal storage to a 4 TB SSD, partition the drive into 2.7 and 1.3 TB partitions. Set up the TM backups on both Macs.

2) Set up two Volumes, one for each Mac. Since volume containers are resized dynamically, when you set up the TM backups on the two Macs you’ll want to select a maximum size for each. If you do not, then if one of the Macs has more file activity than the other, it will ‘outcompete’ for space on the external drive.

Personally, I use the first option to back up 5 Macs in the house to a pair of 4 TB SSDs (I swap one offsite every week). It’s been working fine for several years (and worked fine before that with an HDD, though that was much slower).

In Disk Utility, Add an APFS Volume to the drive for use by the second Mac.

Two computers to be backed up to 1 Drive

All the Eggs in One Basket and the Drive Fails, which they do

To truly protect your non replaceable Data

Have a 3-2-1 Rescue Plan in place and always current

3 Backups using 2 methods and 1 off site incase of natural disaster or un-natural disaster.

Each of the above should be done to a Dedicated Single Purposed External Drive 

Below link is intended to augment what TM Backup does 

https://bombich.com