Imovie splitting-, fragment disappears.

Great! You got your answer.

iMovie uses the word "split" to mean to cut a clip into two or more separate fragments. iMovie uses the word "cut" to mean to remove the clip entirely, i.e, to make it disappear from the project timeline. You can put it back into the project timeline by doing Edit/Paste or Edit/Undo Cut.

-- Rich

I've not seen any reports of a clip disappearing when doing a split. Possibly a corrupt preference is causing that. Try deleting preferences. It is a safe procedure that will not cause loss of data or disruption to your projects. To delete preferences open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects.

-- Rich