A cheap test is to slip in a blank drive and see if you can restore. If you can't restore, the backup is no good.
2. You can't tell since any good backup will only backup used disk space and might compress along the way.
Am using Windows 7 now. I have a disk partitioned into C and E drives. Just did backup to external drive.
1. Can I be certain that all data from both drives was backed up?
2. Why is the size of the backup on my extrenal disk around 50 GB when the combined total of both E plus C is around 80 GB?

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