There is a saved config file that tells it what type of partition to be expecting. If you change the drive out without resetting this config file, it may cause problems.
Look for global config files (I think in a separate directory on the SD card than the /apps/usbloadergx directory, and delete it. Rerunning GX will then reset the config file with a new set of defaults, and it might work.
It is also possible that the drive isn't compatible with the Wii. we have a collection of hard drive model numbers known to work (and known not to work) here: http://www.wiihacks.com/recommended-...lity-list.html
Is the drive USB 2.0 or USB 3.0? If 3.0, you may need a set of v10 beta cIOSs to increase compatibility. I'm not completely sure about which ones to choose, but there is a cIOS guide floating around the site. Search for it.