Drive space on my Retina MBP disappearing?

How are you reading the available space?

Do you have Time Machine turned on? If so, it will show local snapshots taking up disk space from the Storage tab. Even if you delete, say, 40GB, those snapshots will linger for exactly one week. It's nothing to worry about - that space will be gone in a week and if you need more room on your disk, the local snapshots will give way to the new data.

Clinton

Here's a good article on local snapshots - http://pondini.org/TM/30.html.

If you delete a good deal of files on your MBP, your internal drive (when Time Machine is enabled) gives you one week to decide if you want to 'recover' those files or not - they are not a replacement for your TM external backup, just a temporary storage space that gives you a chance to recover files if needed.

Look around the Pondoni site - it's my 'go-to' site for all things Time Machine.

Clinton

For a more relevant measure of available space, select the icon of the boot volume ("Macintosh HD") in the Finder and open the Info window. The space occupied by local snapshots will be included in the Available total, because that's what it is: available.

“You may notice a difference in available space statistics between Disk Utility, Finder, and Get Info inspectors. This is expected and can be safely ignored. The Finder displays the available space on the disk without accounting for the local snapshots, because local snapshots will surrender their disk space if needed."

OS X Lion: About Time Machine's "local snapshots" on portable Macs