I've formatted my 250GB Slim's drive using the serial number for the console, but this process only took about 30s and even though the drive shows 228GB free it still has all my old file info stored in there when you click on the drive again in Storage. e.g. 77GB in Games & Apps, 9GB in Demos, etc.
Click on one of these and all the files are still showing in each section but with yellow exclamation marks next to them, plus they now won't delete manually it says there was a problem.
I've run the format twice now but everything still shows. Any ideas? I'm selling it tomorrow and don't want any info on there.