"The wonderful thing about standards is that there's so many of them to choose from."

Yes, what I was asking was picking a single "PC standard" that isn't encumbered with patents, and suitable for embedded device. If by "standard" you mean FAT, it's currently being obsoleted by NTFS, and in the future to be replaced with WinFS, except these are way too heavyweight to make sense in such a simple device (and are patent-encumbered.) ISO-9660 seems to be a nice open standard, but it's read-only. UDF (packet access to CD) is rather complicated, designed with "hard to delete selectively" CD-RW in mind. SMB over TCP/IP seems to be free but horribly sophisticated if you don't really need to deal with network, especially for embedded devices...
What other standards present in Windows could make sense as interface to communicate with '51? There are still a few left I have no idea about (TWAIN, PCIMCIA), and probably a few I have missed. Or maybe there could be a way around the patent - i.e. making the device completely unaware of the filesystem, and providing disk-like access mode, so you can format it to any filesystem you wish? (except then it will be -only- storage device, and it wouldn't be able to process the stored data in any sensible way)?