Are you trying to use a USB device to play your games off of or to run homebrew applications?
I'm going to assume it's for playing games since you have a black wii and wouldn't pirate any other way so things you should be aware of are that the hard drive needs to be formatted to WBFS which is a special file system that will hold only Wii games and nothing else. (You could also technically now have it formatted to FAT and put games on the FAT partition as .wbfs files, but I'm not sure how noob friendly that is at the moment. Could be just as easy, but I haven't really bothered to keep up with it to know.) So if you plan to use your hard drive for other things as well as your Wii, look into the method of using a FAT partition, otherwise just make it a dedicated WBFS drive or you could just split it into two partitions and format one to WBFS and keep the other as w/e you want.
Another thing is that the hard drive should be plugged in the USB port that is closer to the edge of the Wii, not the one in the middle (unless again something I haven't kept up with changed that). So if you're looking at the Wii from behind when it's standing up on the stand, it's the port on the right.
I suppose it is also possible that if you didn't properly get your cIOS installed that it wouldn't be able to read your hard drive, but it's a pretty damned easy guide to follow so I would lean more towards it being something like using the wrong usb port or an incompatible hard drive or something.
Edit: double checked on my Wii after reading above post, it IS the right side port when looking from the back. Also even if your hard drive isn't properly formatted, if it detects it, it should ask to format it itself or something at least (unless that got changed with FAT partition stuff too).