Question / Problem
A user has a 148 GB hard drive, showing with 49 GB free space. Treesize claims he is only using 58 GB. Why the difference?
Answer / Solution
The Windows Explorer shows the space that is physically allocated on the drive while TreeSize shows the space that is occupied by all files under a certain path. Please make sure that you have the view option "Allocated Space" activated when you are interested in the physically allocated space.
A possibility is that not all parts of the drive could be scanned due to access restrictions. Therefore it is highly recommended to run TreeSize with administrator privileges. Turning on the Option "Track NTFS Alternate Data Streams and NTFS Hard Links" in the Options dialog may result in more accurate results but slows down scans. If a drive letter points to a sub-folder of a network drive, the allocated space (correctly) reported by TreeSize Professional may also be much smaller than the physically allocated space on this drive reported by the Windows Explorer because possibly the whole drive is not accessible through the network.
Beyond the space that is needed for storing the files itself, additional space is used for storing management data like the File Allocation Table of the file system or the boot sector. It is not possible to free this space with TreeSize Professional or any other tool. This is usually 0.5 - 2% of the occupied space. Another possibility is that you are using a Software RAID - like Windows offers it - which spreads the data with redundancy over several disks. These disks will appear as one logical volume and the failure of a single disk will not cause any data loss. But for storing the redundant information additional space is needed.