Not necessarily. In incremental if day 2 is corrupted you lose those files (unless they were also backed up on day 3) but day 3 would be fine.
Look at it this way. If you do a full backup and 5 days of incrementals to restore you would restore the full backup then day 1, then day 2 then day 3 then day 4 and finally day 5.
In differentials you would restore the full backup then day 5. No need for days 1-4 as they are contained in day 5s backup.
hope that helps.
You are confusing incrementals with differentials. An incremental includes only data since the previous image, be it a full image or an incremental image. With incrementals, all copies have to be present and uncorrupted for full recovery. If day 2 is corrupt, you will only be able to restore with the initial full image and the first incremental. You will not be able to use day 2 and any day after that.
Differentials, on the other hand, include all data from the last full image. When restoring, only the initial full image and the last differential is used. If day 2 is corrupted, it doesn't matter since only the last one will be used. Differentials are safer than incrementals but take up more space since each one is larger than the previous on. However, full images are much safer than either incrementals or differentials.
I'm not confusing incrementals and differentials in a file based backup. I use Cobian for my data backups and if day 2 was corrupted due to bad sectors or corruption during file copy process, encryption, compression etc, those files would not be usable but everything thereafter would still be usable.
In something like Acronis or macrium you would need all copies to work for incrementals. That's one of the reasons why I don't use imaging for my data backups.
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Windows 7 Pro 64-bit
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