What is this value?
This value is the difference between the file space used by the pool (or individual mount points) and the total space used as reported by Windows. Now for some users, this value may not be zero, this can indicate one or more of the following:
1) There are files and folders outside of the pool or mount point folders.
2) There are files in the Recycle Bin of the individual drive(s). Important, emptying the Recycle Bin of a pooled drive does not clear this out as the drive's Recycle Bin which sits outside of the pool (or mount point folders).
3) There are files in the System Volume Information folder of the individual drive(s). Again this sits outside of the pool (or mount point folders).
4) There are no (or very little) files outside of the pool or mount point folders, however, if there are a large number of small files in the pool or the drives in the pool are using a non standard cluster size (aka sizes beyond 4 KB), this value can be greater than zero.
How this value is calculated and display
When Drive Bender performs a file system health check, it also audits the files in the pool and records the true size of each file. Now it is important to note the word "true" size. When a file is stored on a drive, it does not simply occupy a fixed number of bytes equal to the file size, it occupies a number of clusters that is at least equal to the file size. By default Windows uses a cluster size of 4 KB (4,096 bytes), so a file that is 5000 bytes in size, real occupies 8192 bytes, which is 2 clusters (2 x 4 KB). The remaining space not physically used by the file, in the case 3192 bytes (aka 8192 - 5000 = 3192), is known as "slack space". This slack space cannot be used and as such Windows does not report it as free. For example, if we store 100,000 of the 5,000 bytes files previously mentioned, for a total of 819,200,000 bytes (781 MB) used, we would end up with 3,19,200,000 bytes (or 304 MB) of slack space. When displaying the non pooled value, Drive Bender takes the total files size it calculated (which is the true size and does not take slack space into account), subtracts this from the used space as reported by Windows (which does include the slack space). The end result can be a value larger than expected, so the Drive Bender Manager attempts to allow for the slack space by assuming an average 1/2 of the default cluster size is wasted per files stored, and adjusts the displayed value accordingly.
Does this value mean anything?
The short answer here is no, this is simply an indication of what may exist outside of the pool... in fact other than displaying a chart in the Drive Bender Manager, this values means nothing. The free space reported is what is actually available.
Should I be using a different cluster size?
This is outside of the scope of this article, however, we do not recommend using a cluster size other than what is recommended by Windows. Using a smaller size can increase the number of files that can be stored on a drive, but also increase the access time for each file. Alternatively using a larger size can improve file access time, but reduce the number of files that can be stored. For more information on cluster sizes, see - https://support.microsoft.com/en-us/kb/140365