I've been working with SanDisk's V-Mate "digital" video recorder. One frustration, after recording two hours of home video off my old camcorder, is that the video file appears to be just 0MB in size (empty), yet Windows reports the 4GB SD card is "full."
I finally found the problem: after pressing [Stop] on the V-mate's remote control, wait for a minute or so -- however long it takes for the red record LED to turn off.
It takes that long for the V-Mate to close the file. (I dunno why, but maybe it has a slow processor that takes a long time to deal with the 3GB MP4 file.)
Once the red LED is off, you can safely remove the SD card from the V-Mate for whatever other purpose you need it for.
I timed it: it takes the V-Mate just under 2 minutes to close the MP4 file. That's a long time!