Just wondering if anyone else is having problems copying MP4 video files for use on the N900?

Basically, the files play fine on my Laptop from my hard drive, but when I either drag and drop them onto my Micro SD card, or cut and paste them across they won't play.

I know it's not a problem with the phone as I've inserted the Micro SD card back into my laptop and they won't play on my laptop either. I also tried copying onto a 6GB card I have used in the past and I get the same issue - "Windows Media Player cannot play the file. The Player might not support the file type or might not support the codec that was used to compress the file." Thing is, the original MP4 that I'm coping plays just fine!

Any ideas??