long time no chat

I have a strange problem. I want to copy a 4.3GB .rar file to my USB backup disc. Iomega 500GB ac powered drive.
Now it tells me that there is not enough disc space. But there is! I have 100GB free.
I delete 9 GBs of files - still tells me there is not enough space when I try again.
However I can copy a 13GB directory with all its contents to the drive. So the space is obviously there!
Also I have no problem copying the 4.3GB .rar to any other drive, even to other PCs on my network - so the file seems to be OK, too!
And I can copy 2BG .rar files to the USB disc. So:
a) disc space is obviously available
b) file can be copied to other discs ( I also extracted it) so it's fine.
c) I can copy 2GB files fine, but 4GB files it tells me there's not enough space
Go figure!! Any ideas wth is going on there?
Cheers

KC
P.S.: I've now tried different files, .rars and ISOs with 4GB or more and I get that same error always