I think I read somewhere that it was a samsung HDD (sadly not a SSD).
I was thinking about doing the same thing to my NAV/Music Box but I figured that since everything is SO integrated (Radio/SAT/TEMP/NAV/M-BOX/TouchScreen) I'd be hard pressed.

I've read about ppl disabling the parking break safety and running a small computer (or of sorts) through the provided AUX composite inputs. Disabling is safefy feature is illegal to do though.

There is actually a product called CAMP2 that is made to work with an AUX input to provide engine information from a diagnostic panel under the passangers feet area.

As for more space, the Music Box can rip songs in two bit rates. It is factory set at the higher bit rate which you can change. Also, connecting an ipod/itouch/iphone is nicely setup on the touchscreen. It will not charge the itouch or iphone while plugged in without an adapter but work just the same. (you can buy one at Best Buy or many other electronic stores)

Regarding the 512 song limit, this is just a pure guess, but maybe this is a folder limit?? In other words maybe you can put the songs on the card and segregate them at 512 songs to a folder? Again this is just a guess because I haven't even looked at whether the CF reader handles subfolders. Sorry if this is a dumb idea...

Well check MYG37.COM - INFINITI G37 (SKYLINE) FORUMS, GALLERY, CHAT, AND MORE. and you will find a long thread like 17 pages of someone taking the hdd (same as in the Z) and hooking it up and trying to upgrade it or reading the files but they hit a dead end when they found that the files for the system are tied to the specific vin of the vehicle. Will probably be the same for the Z since it is the same exact system. Actually in the G the hdd has the nissan badging on it.