if the HDD doesn't have an AC power plug you are going to need a Y-cable to plug it into two usb ports at the same time because the wii does not have enough power coming out of one port.
I got it to work without one, it loads up all my Wii and GC games fine.
I'm having a problem with the ModMii nands working, however. I hacked this Wii several years ago with ModMii and everything ran perfectly, and all those hacks are still on my SD card. So I figured the only thing I'd need to do is recreate the nands with my WiiWare and VC games on them, then add them to the hard drive. I made one for WW and one for VC, with all the hacks installed and default options checked, and none of the games I installed worked. Deleted those nands and tried again, and I'm still having the same problem. I'm selecting the proper channel paths and saves for each nand, and emulation is set to full on both (which worked last time). Not sure if I need to do something else in ModMii to download and initialize those nands, or if the bundled ShowMiiWads 1.5 is unpacking the WADs incorrectly. I doubt that the WADs are corrupted or fragmented, since they are the same ones I used last time. It shouldn't be a power consumption issue if I can run Xenoblade Chronicles from my USB-powered external.
=\ Kinda stumped.