Time Machine not enough space? Don't overlook Dropbox

For months, I've had trouble with my Time Machine backups, regardless of operating system. The error message was that "the backup could not be completed" due to a lack of drive space. I have a 1TB drive attached and only 300GB of files to back up.

I tried several options, including repairing the disk drive in Drive Utilities, erasing and starting over, etc. Nothing worked.

But last week I finally got to the bottom of the problem. I have a Dropbox folder on my hard drive, and even though I'm using Dropbox's Selective Sync (so it's nearly empty), Time Machine must have been trying to download everything in my Dropbox (the way it does when you click on one of the virtual links in the Dropbox folder).

All I had to do was exclude that folder from my backups, and everything is working perfectly now. I would imagine the same situation could come up with any cloud storage app that creates virtual links which are populated when you click them.