Is this something that needs to be done once, to "fix" a design oversight perhaps? Or is it something that you need to do on a regular basis, for instance to add summary data (for instance, the number of data records) to the beginning of the file?
If you need to do it just once then your best option is just to accept that a mistake has been made and take the consequences of the retro-fix. As long as you make your destination drive different from the source drive you should be able to fix up a 500GB file within about two hours. So after a week of batch processes running after hours you could have upgraded perhaps thirty or forty files
If this is a standard requirement for all such files, and you think you can apply the change only when the file is complete -- some sort of summary information perhaps -- then you should reserve the space at the beginning of each file and leave it empty. Then it is a simple matter of seeking into the header region and overwriting it with the real data once it can be supplied
As has been explained, standard file systems require the whole of a file to be copied in order to add something at the beginning
If your 500GB file is on a standard hard disk, which will allow data to be read at around 100MB per second, then reading the whole file will take 5,120 seconds, or roughly 1 hour 30 minutes
As long as you arrange for the destination to be a separate drive from the source, your can mostly write the new file in parallel with the read, so it shouldn't take much longer than that. But there's no way to speed it up other than that, I'm afraid
