philippe99 will be better able to answer your questions. But as a note: 3.5 GB available is too little!!! All mac osx programs use virtual memory (VM) where they store temp memory to the hard disk drive. LW also needs space & the longer it's on the more VM it needs.

Also, external drives can be a little slow at times. So saving to an external drive from LW 'might' be an issue. (But might not also.)

If I were you, I'd free up more space on your HDD. Try to have over 10 GB available without programs running. You'll need to evaluate what needs to stay & what can be moved over to the external drives. Likewise I'm considering moving much of my iTunes library to external drive. I've already moved it off the OSX partition & HDD I have onto another internal drive.