I have been plagued with these constant messages on my MAC over the last couple of months. I dutifully followed all the common suggestions to clean up and recover more space.
I deleted all unnecessary video files, deleted trash, even moved my iTunes library to a separate drive. All these helped....A LITTLE...FOR A WHILE.
Yesterday I was down to 5 GB of free space on my start up drive! This morning I actually watched it go down to ZERO! Something was eating up my drive space..I was on mission to find the culprit or culprits.
I downloaded a nifty FREE little program called OMNI SWEEPER
This allowed me to see into the inner guts of my system to see what was taking up and eating so much space.
#1 was GMAIL
#2 was Adobe Raw Caches
I found in users/smathia/library a folder that contain over 20GB of "recovered messages"
Even after deleting the folder, Mail recreated it and continued downloading the message from Gmail. To solve the situation, I opened the Mail Preferences (Command + , ) and chose both my GMAIL accounts, I clicked on the Advanced tab and changed the "Keep a Copy for Offline viewing" setting below:
I am one happy camper now!!!
40GB of free space