Humax Foxsat HDR external har drive problems

Hi

I connected an 80gb HDD to the rear usb socket on the Humax HDR.

However, it doesn't show all of the folders on teh mdeia list, it shows about 7 instead of the 30 that are on the drive.

I am missing something? Is there a certain format/structure required?

The drive works perfectly when connected to either a Panasonic blu ray or a Sony blu ray.

Thanks for your help.