According to GTPlanet, there’s a bug in Gran Turismo 5 that’ll cause your save game to be unusable. For reasons only known to Polyphony, the save game grows each time it’s used (or saved), and once it reaches 4165KB in size, it can no longer be used.
What happens? Well, each time you load the game the save game grows in size by a few KB, but the museum cards use up much more (100KB or so) – once it reaches that mythical size, you’ll get this (incorrect) error which prevents you from using that same file again:
Insufficient free space on hard disk drive (HDD). At least 32MB required. Please exit the game and free up some space. 0×00000015 
GTPlanet are advising that gamers don’t use the museum feature until this nasty bug is patched out of the game, but there’s not a huge amount that can be done other than keeping an eye on the size of the install file. Once it’s just over 4MB, it’s done for – a strange one indeed.
The update is due out next week (on the 18th) and will also introduce Performance Points, a carry-over from Prologue used to benchmark cars so it’s not just about horsepower and weight.