Backup mystery

[nekonoshucchou]

My main external drive connected to a late 2014 27” iMac, running under Big Sur, is a 12TB HDD containing about 6TB of files.
This HDD is backupped with CCC to three different 8TB HDD’s.

So in principle all these HDD’s should all have exactly the same content, about 6TB.
But there’s one dissident that contains 7TB.
A mystery, because all disks were formatted in exactly the same way, and are backupped to with exactly the same CCC settings.

Does anyone have a clue what could be the cause of the mystery?

 

[Wando64]

If you have SafetyNet activated on your backups, and you did these backups at different times, the first backups will have more data stored in the SafeNet folders.

Protecting data that is already on your destination volume: The Carbon Copy Cloner SafetyNet | Bombich Software

bombich.com

 

[nekonoshucchou]

That could be a cause indeed.
However the safety net is activated on none.

 

[Wando64]

OK not SafetyNet, but I still think it is probably linked to the backups having been made at different times.
Keeping in mind that when you look at the size in TB, the figure will have been rounded and the actual difference might be considerably smaller that 1TB.

 

[nekonoshucchou]

Thanks much for your input.

CleanMyDrive2 shows the difference is about 1,4TB.

 

[nekonoshucchou]

The backups are on a daily consecutive basis.

 

[mick2]

Are you retaining daily snapshots on each CCC drive? If so (and the drives are being rotated daily) this could account for a difference because each drive would have a different set of daily snapshots on it.

 

[nekonoshucchou]

Each backup procedure has exactly the same settings like in the underneath screenshots.

 

[bombich]

Select the task that specifies the outlying destination, then click the "Compare" button in the toolbar. CCC will show what the differences are between the two volumes. If the Compare report shows equal content and disk usage (in other words, if it doesn't display the disk usage discrepancy), then the difference is likely in a snapshot. Click on the Destination selector and choose "Manage snapshots on '{volume name}'" to see the snapshot disk usage and retention settings for that volume.

This may be helpful too:

Common explanations for differences between the source and destination

Mike (developer of CCC)

 

[nekonoshucchou]

Thanks for the info!