Difference between disk free space and used space

During the verification of free space on your hard disk, you detect that the amount of space free mentioned by the computer system is very much smaller than the real space available.

Explanation

Hard disks are divided by allocation units called « clusters » which however represent the minimum size occupied by a file on a hard disk.

Moreover, an operating system uses blocks compiling several sectors (between 1 and 16 users). A tiny file (cluster) will however occupy several areas on your hard disk.

Clusters can occupy a considerable space size depending on the file system used and the overall size of the disc.

Knowing that a cluster can contain data as a single file, when the system saves a file, the data is written successively in clusters empty. When the first cluster is full, the second cluster is used, and so on.

It is evident that a cluster can contain same data as a single file and when the system saves a file, the data is copied in empty clusters. Once the first cluster is full, the second is used and so on.

The finest way to explain the above issue: Taking an example, if the clusters’ size is 4 kb, when a file of 6 kb is saved to a cluster. The rest 2 kb will be saved on the following cluster.

The file despite occupying a tiny space in terms of size may however result into a considerate size if we deen on several files.

The best would have been that all file size equal to cluster size to manage the size capacity.

We should not forget that the operating system also uses part of the disk space to manage all files ((FAT structures, structures MFT system NTFS Alternate Data Streams, etc.)

Those data are however crucial for managing your operating systems and is not considered as space occupied.

How to reduce the loss

It is recommend to reduce the numbers of small files, i.e, if you have a folder bearing small files not often used, the zipper will save space.

NTFS manages more small files in large number specifically on high capacity hard drives. It is therefore more genious to use NTFS rather than FAT.

The NTFS file compression can be used to reduce the space occupied by files. It is indeed to relevant to think that if it reduces the performance, It even reduces the space occupied.

Moreover it is not recommended to compress files used currently and never compress system files.