It's still running and no crash so far (fingers crossed).
Might be worth putting in a call to check available space before each write, and throwing up a dialog if the drive is getting below some threshold, with the option of saving the DB at that point, so that it can be resumed after a clean up.
When the crash happened it lost all DB info it had done on that update - which in my case was 1/2 a day.
Edit - left it running all night - no crashes and got all the MAME media. It was the HD space that was the underlying cause.
Hopefully this will help others who encounter the same problem

Do you remember the site "MAME ROM Links" ? - That was me way back then.....