Excuse the double post but I just feel it's worth it (even if only by a minor degree).
I just ran a small battery of tests on my Wii using the Ultimate USB Loader v7.4 (from
http://gbatemp.net/index.php?showtopic=147085 using the forwarder from
http://gbatemp.net/index.php?showtopic=148345) on 2 of my extHDDs (WD Elements and Maxtor Basics). Both drives have split partitions (1 NTFS, 1 WBFS), and both worked just fine under Wanin's USB Loader v1.1 and cIOS rev9.
Firstly, everything works fine on both extHDDs running the USBL v7.4 under cIOS rev9, although for some reason, it would not download any cover images. Running games works fine, video-patching worked (at least it did for Bust-a-move, patching NTSC to play on my PAL Wii). I didn't try Ocarina because I've yet to find a game worth cheating in.
Secondly, everything worked fine when I switched up to rev10. Cover images downloaded fine and I've now got one for each of my games. No ret -1 or ret -6 errors at all.
EDIT: It might also be worth mentioning that Deadly Creatures and SSX Blur Wii did not work either. I'm not surprised, since they've never been working for me, but apparently that will also be fixed in rev11, so I'll be patient and wait.
I've heard in other threads that ret -6 on drives that worked under USBLv1.1 and cIOS rev9 are due to a bad cIOS rev10, so simply reinstalling it from Wanin's blog will make it work (although people who can't use MediaFire, such as my friend on TalkTalk ISP, will still have to struggle to find a working rev10 mirror or simply wait for rev11). For every other drive getting ret -6, then it's likely that the drive really is not compatible with USBL for now (which'll hopefully be fixed in rev11 - no guarantees).
Now all I need to do is find out how to get the HBC to work with my new Kingston 8GB SDHC card... the Wii will read it, but HBC won't... and again, I've heard in other threads that it's due to HBCv1.01 being compiled with an old version of libogc with a shaky compatibility rating with SDHC.