I've just formatted my PSP 512MB memory card, but when I check it on the PSP it says I have 467MB free space. I've checked and everything appears to be wiped from it, apart from the folders. Why is this?

that is system files i believe my 1gig has only 978mb useable.

Yup it is just the way things are formatted just like if you have a 40GB hd you can only use 34GB one of those weird things.

It also depends on whether space is measured in base10 or base2. That, plus formatting and system information, would take up the extra space.

Ok :) I'm selling it on eBay you see (PSP), just making sure.

That extra space is used in the partition tables aswell as to hold information on how the drive/card is formatted etc aswell as noted bad sectors and such.