From FileJockey Software:
DiskSizes 1.6 shows status information about your computer's drives. This information includes cluster size, free space and total size. Files are stored in an integral number of clusters (or allocation units). Disk and file sizes may be based on powers of 1000 or 1024. When 1000 is selected, a megabyte is a million bytes, a gigabyte is a billion bytes and a terabyte is a trillion bytes. This information may be useful when you are storing many small files. Can measure a folder's disk use and display the number of folders, number of files, sum of the file sizes and total disk use. The last one may be relative to the cluster size on any of your computer's drives. It may also be relative to a selected size or all of the listed sizes. In the last case, this information may be exported to a text file.
Version 1.7 added a Used Space column to the Drive Information display and a totals row. This row shows the totals of used space, free space and total space for all of the listed disks or drives, or only for the selected ones when any are selected. To support this feature, it is no longer possible to sort by clicking on a column button.