How long is it taking to transfer that 500MB?

There is no limit on how large a file can be transferred between computers, but there needs to be sufficient space on the C drive for it and some third party file apps might have a timeout which limits how long a file transfer can take place before aborting.

Are you sending this over the internet rather than a local network?

Are you able to time how long from when you click paste to when it stops is?

Yeah, from my experience there’s definitely a limit. I think around 500–600 MB is where it tends to choke, at least on my setup. If you need to send bigger files, you might have to split them up or use a proper file-sharing method instead of pasting.

Thats a very slow speed.  MP4 is capable of saturating 5GBit lans, so 18mbs is extremely slow.  Is it copying from or to a slow drive?

Last night a 571mg paste got through but the next one failed again near the end.However,something caught my eye - MS Onedrive.Tried it and 

it worked,every time.Yes it takes time for the receiving PC to register the file but time is not a problem.So I've got 5gb to play with. 

Thanks to all who offered help,especially Neil Banfield.