At some recent shoots, I've rented a 7D as video B-cam. During one shoot, when I transferred video files off the first card (which came with the rental - rated for video), only 2.1GB of data would transfer to the computer for two different files indicated as 3.98GB on the card. I tried to transfer these clips three different times to no avail - 2.1GB each try. When I transferred files off my own card next, the 3.98GB file transferred completely.

What could have been up with this to prevent it in the future? I rented the same camera previously without any issue.

It might be that the FAT (File Allocation Table) is corrupt, have you tried running a filecheck utility on the card?

Sounds like a corrupt card. Are you using a card reader? Try a different one, or try transferring direct from the camera. Are you able to open and play the truncated files that you end up with?

And this may not help at all, but if you're on Final Cut you could try converting the footage direct from the card using the Canon E1 Plugin rather than actually copying over the actual H264 file. This shouldn't work either, but worth trying.