I just installed a Toshiba MK4007GAL 40GB in my Sony HD5 and it works great! The 40GB drive actually only gives you 37.2GB because its 39,990,689,782 bytes. There are a few things I've noticed that are different so far: 1. You will need to re-register the mp3 player for your computer if you use sonicstage. 2. This may just be my imagination, but I think the HD access time is faster. I'm probably just imagining it. For legal purposes: I TAKE NO RESPONSIBILITY FOR ANY DAMAGE THAT OCCURS DUE TO ANYONE TRYING TO UPGRADE THEIR DRIVE! I will updating this post within the next week or so with step-by-step instructions on how to upgrade your HD5. (I have a lot of exams coming up so I can't really do it right away). Check back for updates!