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Something’s slowly eating my HD space

Somehow part of my files from my USB disk, 40GB of it (exactly the space I was missing), were copied to /Volumes/ and when I mounted the disk it was called "600GB Disk 2", while "600GB Disk" was filled with duplicated data. All that happened at once, slowly, just this morning when I turned on my macbook. I could notice that thanks to Disk Inventory X.

I could actually see those on GrandPerspective, but I thought it was just scanning my USB limited to that folder for whatever reason. On Disk Inventory I could see the /Volumes/ listing one folder as a folder and the second one as a link, like it should be.

Well, looking at that folder, I quickly associated what was in it with my scheduled backup on Carbon Copy Cloner. I'm still not sure why the USB disk was mounted with wrong name, but what happened was CCC store the full path information of the source and destination, so when it tried to do the schedule backup it created the path that didn't exist, and copied everything there - while it should be copying into the mounted volume.

While this is solved this time, what else could I have done to diagnose this kind of issue, for the next time?