Missing operating system after installing UserOS to a USB key.

Having successfully installed UserOS 10.10 to a USB key I have tried to install it on a second, larger drive. The installation process was exactly the same but the key refuses to boot with "operating system not found" error message. Thinking it might be the 16Gb key, I tried again on a 4Gb key but with the same result. Also tried from within the successful USB boot. The only difference was that the original, successful USB key was previously used for a BartPE system (also successful). Online checking appears to indicate the problem is a missing or damaged MBR but several attempts to fix have failed. I have also formatted both new keys with the HP USB tool prior to trying to install UserOS, again without success.