Which bootloader are you using? I thought that the latest version of rockbox already has CF support (according to the CFmod Guide. I am using version r17153-080418)...
Oh, and I'm using a 1GB Sandisk Ultra II CF card, which is in FAT32 format and already has the rockbox files and some music copied onto it from my current H320 HD.
I'm using the bootloader that petur modified.
My understanding (and experience) is that while the latest version of Rockbox does support CF cards, depending on what CompactFlash card you have (as AlexG noticed at the beginning of this thread, as well as others on the
Rockbox forums) you may also need a recent (modified) bootloader. If you just load the latest version of Rockbox on an H300 iRiver as per the instructions in the
manual then your bootloader will be
version 5. (Also see
this thread.) I suspect that if you build the bootloader yourself from the
source (as a binary is not included in the precompiled SVN release) then that may work, but as petur's already tested a modified bootloader with his H320 you don't need to take much of a
risk. (See the threads in the link I gave to the Rockbox forums for more detail.)
The Sandisk Ultra II CF cards may work with the v5 bootloader, though (my Transcend 133x does not), as a number of issues seem to be related to
old cards and new cards (such as Transcend's 133x and 266x, and the A-Data Turbo 266x), but not midrange cards (such as the A-Data speedy or the RiDATA 150x).