At a guess it sounds like P2 cards are FAT 32 file system which has a 4GB file size limit. Usually the format manufacture (Sony, Panasonic etc) has an import utility which reads the card and joins the discrete clips together. Usually via an XML or index file.
I'm starting to think it's how I am bringing the cards in. I have a single Panasonic P2 Card reader. When I bring in the first card the clip is broken into 4 GB chunks. When I bring in the second card, sometimes it reconnects them into the whole clip, sometimes it doesn't.
I don't know how to get around this. I have tried moving the P2 card contents onto a drive and then importing into FCPX and that does other weird things (4GB clips still show up as well as the "whole" 45 GB shot).
Anyone know how to deal with p2 cards and FCPX when spanning cards?