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).
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.
I had to try three drives before I found one that would work (and both the ones that didn't were originally listed as OK on the USB Device List, so don't put all your faith in that). If the suggestions others have made here don't work then you'll have to buy a new one - from what I've read the small portable Western Digital drives have always worked for people.
i'm not sure what build of gx you are using, but it used to be my sole loader, until my drive which had been supported perfectly since i began usb loading suddenly stopped being detected, much like yours. i switched over to configurable loader and its never given me a single issue.