hard drive - Windows 7 disk properties dialog reports more used space than total size of files on the disk

I have two hard drives (same size, same partitions, same model) that have been synchronized using FreeFileSync. They are used to store data, not the system. After sync, the drives should report same free and used disk space but it is not the case.

First drive (used for storage) reports 781 GB of used disk space, while second drive (backup) report only 778 GB. To get that information, I right clicked on the disk then choose properties.

First, I was thinking that synchronising tool was not doing is job (by excluding some files) but using other file synchronisation tools (like SyncToy or DirSyncPro) give me same result (no difference in files reported justify a delta of 2 GB).

Also: if I go to drive root folder, select everything (all files and folders) and right click then choose properties, total reported files size is 778 GB on both drives. If i look closer at the numbers, there is a small difference of a few megabytes between the two drives however.

Notes :

• Computer have been restarted several times.




• Recycle bin have been emptied on both drives (if I go to $RECYCLE.BIN folder, both are almost empty (only a few KB).




• Shadow copy is disabled. vssadmin list shadows doesn't report anything.




• Windows Explorer folder settings have been modified to display system files and hidden files.




• I have run "Error checking" tool (with "Automatically fix file system" enabled) on both disks.

Why is there a difference in used and free space between the two drives? How can I solve this?

Answer

Alternate data streams

Your source files might have NTFS Alternate data streams. If your backup/synchronization software is not aware of them, they won't be copied to the target. There are many tools that can display them, and since Windows Vista you can do dir /r, which displays not only files but their alternate data streams as well.

Different filesystem cluster size

Disk space taken up by a file is rounded up, as a file takes a whole number of filesystem clusters. E.g. if your file's size is 3000 bytes, and filesystem cluster size is 4K, then the file will take 4K exactly. If your two filesystems use different cluster size, then amount of disk space wasted in this manner will be different.

Other rarely-used NTFS features

NTFS has several other features which may or may not be supported by your backup software:

• Compression. If compressed files are uncompressed during copying, they will take more space.


• Sparse files: Special care must be taken when copying these, otherwise they will "explode", e.g. take much more space and not be sparese anymore.


• Reparse points, hard links, and many other features that can confuse your backup software.

Conslusion

Don't rely on filesystem totals. Use some kind of auditing to make sure your files are copied properly. For example, you may try md5deep to calculate hashes and compare files.