In Preferences, Lightroom Mobile, you can tell Lr where you want these files. Alternatively, right click a folder and set it as the folder for these pictures.
Existing photos in sync.lrdata can be dragged wherever you want, just like dragging any other files in Lr.
Thank you for the answer.
Like I said, I already set an external folder as the sync flder in Preferences>Lightroom Mobile>Location
It just doesn't seem that Lightroom CC is taking this into account. It ignores that folder (it is empty now) and creates a sync.lrdata in ~/Library/Caches/Adobe/Lightroom/Sync Data
I suppose it's kind of a bug, as the other caches' configurations are respected (configured like that since Lightroom 4 and imported on each version upgrade).
Thank you for the hint on dragging sync.lrdata, hopefully this saves me a day and half of recreating smart previews, just a bit worried that Lightroom continues to ignore the dragged file and creates a new one.
I'll try again when it finishes creating this batch, maybe this time with a restart of the computer as well.
Sorry, I had overlooked that you had already set up the folder location.
I think sync.lrdata will no longer contain your originals, which I see as the real problem, but now it should be used purely as a cache and is harmless.
After trying again, first by dragging, nothing happened, Lightroom just created a new one and started the process of re-syncing from scratch.
I thought that maybe it's registering somewhere information about the last path used, for I went and deleted the Sync folder itself (from the Cache folder, Photos, MobileSync, etc,just to be sure) as well as deleted all my online data from my profile.
I then un-synced all photos, and changed the Location folder in Preferences>Lightroom Mobile back and forth.
So basically, I restarted from scratch ... And the result? Nothing changed: Lightroom still uses my main hard drive for caching the sync information .