Yesterday someone on Twitch was streaming some of the maps I made. In a couple instances he found solutions that I did not intend. I'd like to fix this. I know I made the level on this computer, but I had reinstalled Windows once or twice since I published that map. That shouldn't be a problem, considering I only reformatted my SSD, and all my Portal 2 files are stored on my separate hard drive.

Anyway, I load Portal 2, and it gives me a warning saying my Steam Cloud files couldn't be synced. Since the files in the cloud were from May 8, 2012, which happens to be the day when the new editor was released, I knew clicking "upload" and overwriting the stuff on the cloud wouldn't have overwritten any of my maps.

Apparently, however, the P2C files did get erased somehow, as none of my Workshop-published maps are showing up in the editor list. Are the Workshop-published maps' P2C files stored in some other location? If so, where is this?

That happened to me once, around 5-6 months ago... I didn't back them up and lost a lot of puzzles. As far as I know the cloud doesn't seem to save the .P2C files.. I guess it is just like our custom instances, models, materials, scripts, ... or even the maps in the sdk_content/maps folder. Remember to back them up.

It's such a huge oversight of Valve, that they don't save those files somewhere that makes sense, like in the "my documents" folder or whatever.

In case you don't know: the location where the files are stored is "<your Steam folder>/steamapps/common/portal 2/portal2/puzzles/<some random number>". It's a good idea to regularly back up that folder since it's not stored in the cloud, as you have discovered.

If your entire Steam folder is on a separate drive, those files should still be there. But if they're not, I'm afraid you'll have to rebuild them from scratch

This happened to me too, although in my case I tried to back them up - the problem is that I backed up the BSPs (compiled maps), not the P2Cs. So at some point I'll have to try and rebuild the maps based off those.