Re; Imovie

Hi..i imported clips into imovie - saving them on my external HDD. Opened project - edited material- closed project- reopen project and imovie clips in project window stated -'source clips missing'. I then copied clips from HDD into imovie events folder - still no response. A MAC technician suggested importing clips onto my computer desk top - then restart computer- then copy them into imovie events folder - still no response. Any clues other then re- importing clips from my digital camera - again.... i have already wiped the clips so no good there... Any clues. ??