Acronis True Image supports file and disk backups. File backup and disk backup are very different in nature. While file backup operates with files, disk or partition backup operates on the sector level.

Hard disks consist of sectors. Sector is the smallest storage unit on a disk, usually 512 bytes in size. Operation system stores files in sectors according to its rules and they are not always straightforward and obvious. This means that the size of all backed up files may not be equal to the size of all data in backed up sectors.

Why does the amount of selected data differ from the data in a backup?

There are 3 main reasons:

1. Files are compressed by the filesystem. NTFS supports native file compression, which allows saving disk space.
2. Sparse files. Such files have so-called sparse areas which consist of zeroes. Operation System does not allocate sectors for such areas until they are actually written by an application. In this case files may have larger virtual size than sum of sectors they reside in.
3. Hard links. This is a way for filesystem to use the same sectors for several files with different paths.

In all these cases file sizes are greater than the size of data in sectors these files reside in. When a system disk is backed up locally as a disk or a partition, Acronis True Image backs up sectors, not files. Such cases may lead to unclear result like one on the following screenshots.

’Data size’ is way greater than ’Data selected’

• ’Data selected’ is disk used space. It considers only disk level (sectors occupied) and does not respect any excludes (neither True Image, nor VSS excludes).
• ’Data size’ is the total size of all files in the backup. It is equal to the sum of all sections of the statistics line.

’Backed up’ is less

• ’Backed up’ is the size of data backed up sector-wise in this slice.
• ’Data to recover’ is the number of bytes that will be restored from this slice. The number is equal to ’Backed up’ because it was a Full Backup (if it was an Incremental slice these numbers would differ).

The size of the .tib (or .tibx) file is usually less than ’Backed up’ because Acronis True Image uses its own compression.

'System data'

Acronis True Image marks as 'System' the contents of these folders on any disk selected for the backup:

For Windows:
    \Windows
    \Boot

For macOS:
    /bin
    /etc
    /home
    /net
    /opt
    /private
    /sbin
    /usr
    /var
    /Library (excluding /Library/Application Support)
    /System

For example, a "Boot" folder can reside on a hidden EFI partition and you are not able to check its size from Windows Explorer. Or you back up several disks, and one of them contains a folder named "Windows" which is not C:\Windows, but this data is still marked as 'System' in Acronis True Image.

Moreover, the folder could contain hidden files or files that change in size (for example, system logs) over time.

Discrepancy is expected in these cases.