It may not be the card, but this type of problem usually is.
I had frequent drop outs transferring files to from the zaurus using the transfer utility using both serial and very rarely usb.
If you are using usb, try a different usb socket, also try and plug it directly into the PC and not via a hub (if that is how it is connected).
You could also try copying the files to internal flash and moving them to the sd card later.
If you have the card reader option in the rom (cxx0 device, or backported driver) then try that as well.
Peter.