I cannot symlink MobileSync

What you are trying to do worked many years ago but has not worked for a number of years. I don't know why, and I was also interested in doing this myself for the same reasons as you.

If you search carefully in Discussions and elsewhere, you will find many who tried but none who succeeded (as far as I can tell).

Some relevant points:

• Even if you find a way to do this (symlink), it might stop working with a MacOS update or upgrade in the future. Because it isn't supported.
• More importantly, you really need a reliable assurance that restoring with this scheme back to a new or existing iPhone has to work. How would one test that. The stakes are high because if it doesn't work, you have lost everything.
• iMazing is an attractive product. But it does not (to my knowledge) enable complete seamless restoration of the entire iPhone with all is customizations and installs like Apple's backup/restore does. But iMazing can backup and restore most if not all of the key elements of what people store on their iPhones.
• I used to do the backups on my Mac and then copy them to an external drive, then delete them from the Mac. But that requires that you have enough space to at least do the backup on the Mac. I also never tested that copying them back to the Mac would then work for a restore.
• In the end, I switched to iCloud backup. That completely solves the issue of not enough Mac storage. I have restored many times from iCloud to new iPhones when they were acquired and it has always worked seamlessly. No external drives to keep track of, no worrying about them failing, or even backing those drives up. If you factor in the cost of purchasing and backing up external drives, it isn't clear that iCloud storage is cheaper or more expensive. Also, iMazing is not free either. Any solution that works reliably will not be free.

unixnerd09 wrote:

I need to back up my iPhone on an external hard drive because there is not enough storage on my Mac.

I tried to symlink the MobileSync directory to the external hard drive,

I would look at imazing

iMazing | iPhone, iPad & iPod Manager for Mac & PC

or simply free up storage on your Mac—

How to free up storage space on your Mac - Apple Support

Free up storage space on Mac - Apple Support

Finder>Locations Control click to manage ... if you have a bunch of old backups remove them(?)

Back up iPhone - Apple Support

unixnerd09 wrote:

...Then everything worked like it was supposed to work, ls, and ln -s, and the backup.

I see your point with sim link may not be supported in the future.

It appears that everything worked, but will restoring from that backup work? How can you check or verify that? Backing up and restoring is done through Finder now, which means the OS has to support it. Will restoring from that symlinked file work for a new iPhone or an older one that had to be reset?

I think symlink is and will be supported, but the iPhone backup and restore might work properly only under certain configurations. Your backup completed, but how do you know it is a viable backup?

unixnerd09 wrote:

Good question, I can only say it appears OK as the "restore" button is functional. To be sure I should reset the phone and try it but only after doing a standard one-time backup on another Mac or the cloud.

I assume a reset phone is the same as a new phone as far as the restore from backup is concerned.

I guess I would strongly suggest backing up to iCloud. I am not a fan of experimenting with backups, you have an unconfirmed experiment there. Problem is, without a viable backup, when your iPhone needs to be replaced (they all do eventually), you need a bulletproof backup.

I also would not do other experiments with resetting your iPhone. What if something goes wrong? I do believe that a reset iPhone is basically similar to a new iPhone in most respects. I would not experiment, however.