You'll have heard this one before, I'm sure, but always back up your stories. Preferably twice, and in different places.
I wrote a 50,000 word draft during NaNoWriMo a couple of years ago that I was really quite pleased with and was looking forward to refining. I'd even managed to rack up a 10k day while I was writing it - something I'd never done before ever!
I'd got it safely on a memory stick, but then disaster struck - the memory stick broke! It wasn't corrupted, and as far as I know all the data is still on it, but two of the connections inside broke free of the board and I can't get it to work. I even took it to a computer shop and asked them if it could be fixed, but they said no.
So I've now learned from bitter experience that if I write something that I want to keep, back it up. Save it on a hard drive and on a memory stick and on something like Dropbox or Googledrive. Failing that, just emailing it to myself is better than nothing. But always back up!
• Challenge for today - back up whatever you're working on!