Backup via Time Machine to external drive (on a different Mac)

Time Machine should have a dedicated target volume. Backing up multiple Macs to a single external volume is a recipe for trouble.

Reformat the drive (GUID, APFS, not encrypted), and create two (or more) volumes in Disk Utility, e.g., TMB_iMac and TMB_MacBook. Size them appropriately for the two Macs (ideally a TM backup has 2-3x the drive being backed up, but less is ok if needed it just means you can’t go as far back in Time). Then you can set up backups (encrypted is best) from each Mac to its respective volume.

I do this with 5 Macs in the house – a 4 TB SSD with 5 appropriately-sized volumes. (Full details, I have a pair of those and I connect one them every week so I can keep swap it out with the other stored offsite; those are my secondary backup, the primary one is to a local 2x10 TB NAS with volumes for each Mac TM and 6th volume as a general file server.)

Are you trying to use Time Machine to back up one Mac to an external drive physically connected to the other Mac? Sorry I missed that in the title, but no. You need to connect the drive directly to the Mac it’s backing up. Connect the drive to the iMac to back up the iMac, unplug it from the iMac and connect it to the MacBook to back up the MacBook. That’s how TM works with regular external drives. Not shared over a network.

The only way to replicate what Time Capsule did in terms of wireless TM backups is to use an NAS. Easiest to get one that offers turnkey Time Machine support, like Synology (which I use) or QNAP.

Some people ask about connecting an external drive to the USB port on their router. That works as a file server but I have yet to come across a router that supports TM protocols, which is what would be needed.

kickns wrote:

Well, that explains that. Guess I didn't understand how that worked (or didn't). On to Plan B (whatever that is; maybe NAS, maybe just a separate drive for the iMac; it doesn't get heavy use and not that many important files on it). Thanks for the help.

If the drive is large enough, add an APFS volume to the drive and set that as the network backup. Attach the drive to your iMac and connect the MacBook over your network.

Time Machine on Sequoia won’t let you just “share” a drive from one Mac and have another Mac back up to it. That only ever worked with Time Capsule or a proper NAS that speaks SMB/Time Machine. With a regular external SSD, the drive has to be directly plugged into whichever Mac you’re backing up.

If you want one drive for both machines, the clean way is to create two APFS volumes on it (one for each Mac) and physically swap the cable when you run a backup. If you want true network backups without juggling cables, then you’re looking at a NAS (Synology/QNAP) or another Mac set up as a dedicated Time Machine server.

iamshivam wrote:

Time Machine on Sequoia won’t let you just “share” a drive from one Mac and have another Mac back up to it.

I wonder why I am able to do it, and have for years

That only ever worked with Time Capsule or a proper NAS that speaks SMB/Time Machine. With a regular external SSD, the drive has to be directly plugged into whichever Mac you’re backing up.

it has always worked, and it still works under Sequoia.

Yes, Time Capsule uses a deprecated drive format (AFP) that won't be supported by macOS 26.