I have a USB hard drive formatted HFS+ with videos on it. The drive shows up fine on my Mac, I can play them, everything works fine.
However, through the PS3 (and Twonky browser) the videos don't show up. If I copy a video from the usb drive to the MB itself it works just fine.
When I tell Twonky to rescan it appears to check usb1-1share1/usb1-1share2 but it doesn't ever find anything. This appears to happen with any USB drive I plug in.