thanks itimpi,
Now that I think of it I might have accidentally mounted it with the unassigned devices plugin during the pre clear, so i stopped the array and made sure everything was unmounted and re added it and now it does work! however I only see 315MB of the 250GB when I add it to cache, I do remember seeing the disk split in 4 parts when the array was stopped, what should I do to get rid of all these parts and reclaim the other space?
Thanks