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recommended storage allocation for backups

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If I have a 1-TB internal hard-disk drive, and a 4-TB external hard-disk drive, how many sector-by-sector backups (including unallocated space) can I keep on the external hard-disk drive?

I just upgraded to ATI 2020, but haven't tested the new version out as thoroughly as I did with ATI 2019.  With ATI 2019, backups would fail if I did not have considerably more free space available on the backup drive than the amount of data I was backing up.  For example, I could not do a sector-by-sector backup of a 4-TB drive to another 4-TB drive.  I am guessing that maybe the reason is because ATI creates temporary files, even if it is overwriting the previous backup?  So for example, if a 1-TB drive needs 1 TB of temporary space to perform a sector-by-sector backup (including unallocated space), then could I fit up to 3 sector-by-sector backups onto a 4-TB external drive?  Likewise, would I need an 8-TB drive to be able to complete a single backup of a 4-TB drive?

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Nicole wrote:

If I have a 1-TB internal hard-disk drive, and a 4-TB external hard-disk drive, how many sector-by-sector backups (including unallocated space) can I keep on the external hard-disk drive?

Question: why do you want to make sector-by-sector mode backups including unallocated space?  This is not needed for any normal Backup & Recovery operation and will cancel out any compression and exclusions set for the task, so the backup size will be the disk size, i.e. 1TB.

If you do this then the max number will be 3 backups - this because the 4TB drive is not actually giving you the full size of 4TB.

For example, I could not do a sector-by-sector backup of a 4-TB drive to another 4-TB drive.  I am guessing that maybe the reason is because ATI creates temporary files, even if it is overwriting the previous backup?  So for example, if a 1-TB drive needs 1 TB of temporary space to perform a sector-by-sector backup (including unallocated space), then could I fit up to 3 sector-by-sector backups onto a 4-TB external drive?  Likewise, would I need an 8-TB drive to be able to complete a single backup of a 4-TB drive?

Sorry but the above is not correct.  Any temporary files created are those created by the Microsoft VSS snapshot service not by ATI, and these snapshot files are stored on the Source drive, not the target drive.  ATI does not overwrite previous files other than those that have been deleted by previous operations and are thereby classed as being free space on the drive.

But to repeat my earlier point: you do not need to be using sector-by-sector method nor including unallocated space in your backups.  This is something I have not needed to do myself in over 10 years of using many versions of ATI.

If you make a normal Disks & Partitions backup of your 1TB drive, assuming that you have used approximately 60% of that drives capacity and are using the default exclusions, then I would expect that a Full backup image would have a size of around 400GB maximum with default compression, and therefore you could potentially store around 9 such backups on your 4TB drive.

I don't want to shy away from sector-by-sector backups of my operating-system hard disk if I can avoid it.  My testing with ATI 2019 did not show any significant difference in TIB size depending on whether unallocated space was included or not, so I have kept the unallocated space in the sector-by-sector backups.  If I want to have a custom configuration, such as for a hard disk with multiboot, a sector-by-sector backup will provide me with the best recovery options.

What would be the smallest size of drive needed to make a single (recurring) backup of a 4-TB drive?  I was not able to back up a 4-TB drive to another 4-TB drive with ATI 2019.  For an external drive with just user files, I don't need to back up sector by sector.

Nicole, the decision is obviously yours to make but even with multiboot systems, I still have never needed to use sector-by-sector mode and was able to migrate to different / larger drives with no issue (system had Win 10 Home / Win 10 Pro & Ubuntu 18.04 LTS on a single 1TB 2.5" laptop SSHD).

With regards to your 4TB drive, how much free space is remaining on the drive, and what options are you using for the backup task, i.e. is this a Files & Folders backup, or are you doing a Disk backup and using sector-by-sector?

Right now, I am using a desktop PC with a 1-TB internal HDD plus three (independently powered) external HDDs of 4 TB each.  I can use one 4-TB external drive for up to 3 Acronis sector-by-sector backups (once a week to go back in time up to 3 weeks would hopefully be sufficient).  The second external 4-TB drive contains user files, and the third 4-TB drive is a backup of the second drive.  I would like to find a way to automate backups of my user files though.  I was considering ordering an 8-TB drive, but if I need a 2:1 ratio on storage for automated backups, then the cheaper alternative would be to partition the 4-TB drive into two 2-TB partitions, and then only do automatic backups for the 2-TB partition with the files that are changed the most often (onto the third 4-TB drive).  The other 2 TB would have to be backed up manually onto a portable drive (such as for archiving static photos).  User files take up a lot more space than system files though, so it's not very cost-effective to need twice as much storage for automated backups versus manual backups (by copying & pasting the files in the file explorer).

The second external 4-TB drive contains user files, and the third 4-TB drive is a backup of the second drive.  I would like to find a way to automate backups of my user files though.

If you use a Files & Folders backup task for your user files on the second 4TB drive, then with default compression and the assumption that the drive is not full with no free space, plus that your files are not already in a highly compressed state, i.e. as with photos, videos, music.  Then you should be able to create a full backup that might allow you to get 2 such backups on your third 4TB drive.

For Files & Folders, you do not need to use a Disk backup and not use sector-by-sector.