[dnanian]

Hi, en. No, you definitely can't do that -- since the internal drive will be active when you're booted from a Sandbox, you cannot partition the internal.

On top of that, you really don't have enough space on your internal, given your data set, to have a Sandbox. It's just too full.

In your case, I'd probably just upgrade the internal after doing a backup (and ejecting the external drive). Make sure everything works well but don't do anything you want to save, just in case you need to restore (from the backup).

Hope that helps.