its reasonably tricky, and will mean you will only be able to read/write to the n900 over the usb cable from a linux machine (ubuntu or similar). So I'm really not sure you want to do this. How about you use 7-zip and split the large movie file up into smaller volumes?

I'm assuming you just want to transfer it somewhere, seeing as theres not much chance of the n900 being powerful enough able to play back such a huge file..