During a shoot always copy the “CONTENTS” and “LASTCLIP.TXT” to a drive to empty cards, never try to re-wrap files on location!
Best thing is to bring enough cards to make it through the day.
If you bring laptop, card reader & HD, make folders in advance; CARD 1, CARD 2 etc.
Lately I’ve seen FCP skip files or abort transfer in the log & transfer window.
You see a red stop sign with an exclamation point.
This is with a fast laptop 3.06 gHz with 8GB ram.
Only after many tries will it transfer certain files.
See this link on Apple
It is too much of a pain to have to troubleshoot and make sure you have all files on location.