While it's waiting, try unplugging your drive completely, and then plug it back in. Some drives, mostly those with external power supplies, don't mount properly when you turn on the Wii and launch GX. If that doesn't work, maybe ensure you've set up the drive properly. It's recommended to format your drive or second partition to FAT32 (you can use the free program called Compuapps Swissknife) before formatting to WBFS. Then use WBFS Manager 3.0.1 to format to WBFS, and add an ISO on your PC first (if possible).
What brand/model of drives are you using? Have they been confirmed to work on the
USB devices compatability list?
If you decide on getting a new drive, I recommend the 500gb Seagate Freeagent Go. I've put three of these drives through every possible test, including homebrew, MPlayerCE, multiple partitions, spindown errors, and they are FLAWLESS.