Hourly backups are your friend. Excel likes the sort of 'this file in invalid' errors, they usually seem to result from the system running out of memory (it's a hog) and consequently when you think you've saved something, you've saved a nothing. Still, it's compact, you can reduce 350 MB to 128 kB this way. It's a toss-up whether the pretend save is an improvement on the 'this file cannot be saved' error (ah, you think, I'll copy and paste this to a new black workbook, but they've thought of that, what's that again Excel, this operation cannot be completed). Computer says no.
