Dm800hd openpli EPG on HDD/USB problems

is there a minimum usb capacity for epg/cooltv. the reason I ask is that with hdd & 2gig usb installed I could not get epg to work, even when I selected HDD for epg, it would not work. I changed the USB to an 8gig and epg loaded fine cooltv works, but why?, that I do not know. Can anyone throw any light on this please