I now ran against exact the same problem of a Wii only working with IOS58 as a cIOS base. Having modded quite a few Wii's I off course went the hard way and reinstalling WAD's using older cIOS versions, using other SD card, reformatting HDD's, changing settings first in USB Loader GX and other loaders. On the way I've got even more confused because also another HDD didn't work anymore which worked on my default setup.
Anyway, after 2 full evenings spending on figuring it out (and having 3 different Wii's available) I've figured out this problem is depended on the Wii edition! My (default) black Wii can see and load games with at least IOS57 (and IOS58) where the 2 white ones can only with IOS58 and will go back to the main menu after booting a game. This is with using the exact same setup (SD card files and HDD games). You should think it is a power supply problem, but I even tried it with an USB-Y cable and still had the same problem on the white Wii's.
Hopefully this info will be spread more (and maybe added in the
trouble shooting guide), as apparently some other USB hardware is used in some Wii's making some external HDD's incompatible on other IOS bases then 58.
Some more info on hardware used:
HDD's: Western Digital 500Gb WD5000ME 4409E with Micro-USB connection and a WD 750GB WDBABM75000ABK.
Incompatible White Wii serial nr. starting with: LEH123xxxx and one with LEH110xxxx both Boot2 compatible so older ones.
Black Wii serial nr. starting withL LEM501xxxx