I never said it was easy and I think I have a pretty decent understanding of what would be required. I just have high expectations! Designing an in car computer with the functionality of the empeg that will fit in a standard DIN slot is not exactly easy either.

Its not just "not easy", it is outright impossible using todays hardware. There are two reasons for this:

1. The operating system (Win/MacOS) is a multitasking system, so writes to different destinations on the same drive might mix with each other. This becomes even more likely with caching systems that accumulate writes before flushing them, ordered not by file but by physical location of the file.
   
2. Because you might have multiple copies of the same file on the device (virtually), you can´t give a correct value of the (virtual) drives size, because the OS really sees say, 10 copies of a file (not knowing that they are copies), but they only take the space of 1 copy. What if you decide to add one more copy of(reference to) that file, but only half the space needed for the file is actually free on the drive? Using emplode, this won´t be a problem, but using the file interface you suggest?

There are even more things. Because of the first point above, the software on the empeg would have to accept the file writes, than search the whole drive for possible dupes (probably using content comparison rather than mp3 tag comparison), remove the dupes and replace them by links to the first copy.
A legal issue is also important to consider: Currently, the empeg is a "write only" device, you can´t get the files back from the empeg without fiddling with the developer image etc. This keeps empeg´s back free from legal issues, as discussed several times before. But if you were emulating a USB drive, you would be able to retrieve the files back (if you weren´t, the whole reason for implementing the file interface would be defeated, I think).
The issue would be completely different, if:

1. the legal issue about fetching files back from the empeg would get solved, and
   
2. one used the same fid-structure on the local disc as on the empeg (which would be just great if players for Windows/MacOS etc. existed that were able to use this layout).

cu,
sven
(MkII 12GB blue now green, #080000113)