Welcome to the Rockbox Technical Forums!
I'd say restore with iTunes on Windows and try installing with RockboxUtility again.
If you already have the bootlader installed, you can just format the SD card to FAT32 using a card reader and put the .rockbox folder on the card and insert it into the adapter. Then the bootloader should load the rockbox firmware. The apple firmware might not work properly then, though disk mode should work. But if you have a 6g iPod which is limited to 128GB, as far as I know disk mode might cause problems. If you stay with Rockbox the 256GB should be fully available.
I mean, that the bootloader needs to be installed first. If the bootloader is installed, you can swap the storage (containing a .rockbox folder) and Rockbox will boot. I don't know if installing the bootloader works without a HDD or a flash card.As I said, as far as I know, disk mode is part of the apple firmware and limits the iPod to 128GB.
You don't need Rockbox Utility to put the .rockbox folder onto the card. Just format the card to FAT32 using a card reader (I suggest guiformat for Windows: http://www.ridgecrop.demon.co.uk/index.htm?guiformat.htm). Download a recent build or the release http://download.rockbox.org/release/3.15/rockbox-ipod6g-3.15.zip unzip it to the crad an put the card into the iPod. Then Rockbox should start.
Page created in 0.139 seconds with 23 queries.