Updates to Files On-Demand on macOS

Liz_Atems_elisatems You might try what Emanuele_Barone did, and clone your internal drive to the external SSD, and just boot from that instead of the internal. When I had a 512GB machine (before I got the MacBook Pro earlier this year), I had to deal with a lot of music software that wanted to put gigabytes of data in /Library, ~/Library, or /Users/Shared. In some cases, I was able to copy that data to an external SSD and point the relevant application/plug-in at the new data location. In some cases, I was able to move data and create a symlink where the software expected to find it, but again, that didn't always work. For the cases where it did work, though, I don't think it was particularly dangerous, as it was easy enough to make sure that the external was connected before running the relevant application (or plug-in).

So, if you really are in a situation where you can't easily move any of the data off of the internal piecemeal, just cloning everything to the external and booting from that may be your best option. If you don't have anything to do the cloning currently (Carbon Copy Cloner or SuperDuper work well for this), you can download SuperDuper from https://www.shirtpocket.com/ - in it's unregistered state, you can do complete drive clones, but can't incrementally update a clone once created. For the purpose of creating a bootable backup, though, that will do the trick. After the clone is created, you can go into System Settings > General > Startup Disk and select the external drive.

The only headache might be finding a spot to store the data that is on your external SSD if it won't all fit on the internal, as the cloning process will wipe the external SSD as a first step. If you can get everything onto the internal before starting the cloning process, though, you'll be fine once you boot from the external drive.