OK. To back up, you'll always schedule Betty Boot to Bertha Backup, using "Backup - all files" and Smart Update.
I'm not sure how the backup actually changes your Sandbox... all the backup will do is make Bertha Backup the same as Betty Boot.
The rest, it sounds like you're right. And, yes -- it is likely that it'll be larger than 80GB. I think you sound like you're in good shape.
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--Dave Nanian
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