i thought thats what happens when you increase the size of the drive. I beleive that the sky+ box assumes the size of the hard drive, if its larger you will eventually reach 100% full but can still record more.
I may be wrong but i'm sure i've read this elsewhere before
do you need to flash your SKY+ box?
http://www.avforums.com/forums/archi.../t-144923.html