I've never gotten it to work either on an S. (still own one, as well as a Zune 30 which is the S's cousin. Same problem).
Not sure if I've ever tried an iFlash on my S. But I have several knockoff cards and even an mSATA / m.2 ZIF adapter and none of them work for S or the Zune 30. Only HDD change that has ever worked for me are those ZIF SSD's like the Kingspec, but those aren't cheap.
Zune, Gigabeat V, Gigabeat S all the same in that respect. Gigabeat X (which I actively use) mostly does not work except the mSATA / M.2 but battery life is quite bad with Rockbox and couldn't find way to fix that so ended up using a Super Talent SSD.
The winner here is actually the F. That thing seems to take everything like a champ.