Are you sure you need a complete filesystem for your logging application? If its only being used for a single log file, you'd probably be better of treating your device as a stream (as in unix). If on the other hand, you have need to store different logfiles and need access to them separately, you will need some kind of file-system.

Look at the stuff as being layered.
You need a USB communication layer to get commands to/from your hard-disk. You will need the command set of IDE or SCSI to control your hard-disk (IDE or SCSI-layer) and you will need the FATxx layer (using your IDE or scsi-layer) to store/retrieve data from your disk.

I personnaly would start by trying the USB stuff, followed by the IDE or scsi stuff (which will use the USB-stuff), and then trying the FATxx routines (using the IDE/SCSI driver).

search for USB,SCSI, IDE, FAT16 on this forum and google. Also try to find MP3 players based on 8051 conntaining either a hard-disk or a cdrom.

Good Luck
Patrick