The issue is not, then, free drive space, which was my first thought. You had tons of free drive space. There was absolutely no need for you to make room on the drive. (When you deleted stuff, you couldn't possibly have removed 900+GB. GB is Gigabytes. 1 Gigabyte on a hard drive is 1,000MB or Megabytes.)
If I am reading correctly, it is 900MB.
Where was this coming from? This really threw everyone off.
Can you post back with the following, which I asked for in my first post.
Open Activity Monitor (in utilities)>System Memory. How much Free is there? What is it showing for Page ins, Page outs and Swap used?
Also, in Activity Monitor, switch to CPU>Active Processes, and see if there is one application in particular consuming a great deal of Real memory. This is assuming all the applications which were open when this message first appeared are still open. It may be one application in particular that is causing this. It may have a large memory leak, which is gradually consuming more and more memory (almost all applications and processes gradually consume increasing amounts of memory...to a point.) If there is one culprit, it may not still be open and this may have to wait until the next time you receive this message.
If you've restarted since and now have fewer or different applications open, the information I asked for above will have completely changed.