I believe the HDD is 40gb. That's what it is in the G. Infiniti stopped doing Music Box (that's what the feature the OP is asking about was called) recently. I believe they were going to have to pay a substantial royalty to someone to keep the feature alive and since so few customers actually use it, they decided to stop (that's mostly conjecture, I haven't heard it from anyone at Infiniti). Of all the trade-in cars that I see, most that have Music Box, have at most 3-4 albums worth of music on them.
You could probably hack the system to make it work, but given the cost of a USB thumb drive, what's the point - since at most, you'd get 9.5gb of space (the rest is taken up by Navi data).