this is to help other people out. My Samsung BC-C8200 HDD Blu-ray recorder wouldn't recognize my external USB harddisk (Packard Bell Silver 320 GB 2,5"). USB-sticks would be fine. I tried formatting the HD in both NTFS and FAT, but no luck. I finally almost gave up, as the manual says only MSC-devices like USB-sticks are supported. This forum contains many threads from people asking why their Samsung TV or other device won't recognize their external USB HD, support replying that they cannot tell which HD works and which doens't, you'll just have to try.
I gave it one last shot, assuming that the USB port wouldn't supply enough current to my HD, which has no external power connector. So I bought a USB hub (this http://www.eminent-online.com/nl/product/342/em1036-high-performance-4-poorts-usb-hub.html one from Eminent in my case) with external power and voila, my HD works flawlessly!
So, I'm not saying that an external USB hub will solve everything, but if you have one or don't mind buying one, give it a try! And if you like, report back here.