I've only had good luck with CF cards (and hard drives, obviously) on an iPod 4G. In both cases I additionally had to disable UDMA to ensure completely flawless long-time operation. Otherwise, sooner or later FS corruption would occur, which becomes especially noticeable when using the DB where you'll be asked to re-initialize it out of the blue.
Yes, CF costs a little more, but for me it was worth it. I actually have the problem that I'd like 512GB of storage space, which simply isn't available at the moment.
Let us know if you find something that works.