Every few days I get notices on my MacBook that it's running or run out of hard drive space. Curiously, restarting the computer will enable me to recover gigabytes of space (this past time, it was able to recover about 2.2GB). However, I can't identify anything in my personal activity that consumed that space.

It's possible that it's a rogue iTunes podcast or a huge software update that my Mac is automatically downloading - would either of these reclaim the space upon a restart?

One possibility that I can think of is that FileVault has some sort of disk leak, allocating but not freeing files. Does this make sense? Is there a tool that I can run to determine where this space is going? Assuming it *is* FileVault, should I try to disable it? What's the best way to turn of FileVault on a nearly full computer?

Warning about answers and new macOS
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For users of Ventura and newer macOS. Apple have increased control of what can be seen by Apps so the apps listed as answers will not work fully without some extra configuration. They will need to be granted Full Disk Access and to see files that cannot be read by the current user will need root privileges - so version from App Store will not provide complete results. See [System data storage is huge](https://apple.stackexchange.com/q/450402/237) (This is not part of the question but I can't think of anywhere else to place this as applies to all answers.)