Panasonic SD Card reading problem

I've just purchased a Panasonic NV GS 400 camcorder which has the capability to save pictures to SD card. When I connect the camcorder to my PC via the USB cable, I can transfer the images without problem (it appears as a removable disk drive in Windows Explorer). However when I try and access the files via an external file reader (by taking the SD card out of the camcorder and inserting it into the SD slot of the card reader), I can see the top-level file structure folders but not the individual files - Explorer gives me an error saying that the files are corrupt or unreadable. When I reinsert the SD card into the camcorder and connect it to the PC, everything is OK. Has anybody else experienced this problem? And is there a way around it? (I want to be able to access the images direct from teh SD card as it saves the hassle of hooking up the camcorder all the time, and I also want to be able to use a HDD MP3 player which has an inbuilt SD slot as a way of backing up my files when I don't have a PC with me). Also will I have the same problem with a Panasonic Digital Camera (such as the FZ5)? Any suggestions? Thx