@ burble
Might be an idea to keep an eye on this post at the development forum when you set yours up.
"Managing a USB hard drive on the Plug."
http://plugcomputer.org/plugforum/index.php?topic=485.0
Apparently the USB socket takes a few seconds during bootup to go through it's self checking state and initialize itself and some External USB drives are not being seen as the kernel has already booted instead of waiting for the USB int string.
.