I have not had any problems with sticks or drives, I have heard there are sometimes problems with sticks but this is the first time I recall hearing somebody has had a problem with the Humax recognising an external drive. Have you tried front and back ports (the rear port is tucked under the ethernet socket)?
My first reaction would be to get another memory stick. I have a Medion 8GB (Aldi own brand) that works fine. I also have a Buffalo Ministation 320GB that also works fine.
Other than that, you will need to install the modified software, set up a virtual drive, and do the OPT+ copy to that (it pretends to be a mounted USB drive). You would then need to use the extended FTP to be able to access the virtual drive (the standard FTP will only allow access to the standard media directories). There is an alternative coming along which will use DLNA streaming to copy files decrypted, but I'm not up to speed on that yet.
Refer to the threads at the top of the list for more pointers.