The "Other" and "Unusable" sizes displayed in the DrivePool GUI are often a source of confusion for new users. Please feel free to use this topic to ask questions about them if the following explanation doesn't help.
Unduplicated: the total size of the files in your pool that aren't duplicated (i.e. exists on only one disk in the pool). If you think this should be zero and it isn't, check whether you have folder duplication turned off for one or more of your folders (e.g. in version 2.x, via Pool Options -> File Protection -> Folder Duplication).
Duplicated: the total size of the files in your pool that are duplicated (i.e. kept on more than one disk in the pool; a 3GB file on two disks is counted as 6GB of duplicated space in the pool, since that's how much is "used up").
Other: the total size of the files that are on your pooled disks but not in your pool and all the standard filesystem metadata and overhead that takes up space on a formatted drive. For example, the hidden protected system folder "System Volume Information" created by Windows will report a size of zero even if you are using an Administrator account, despite possibly being many gigabytes in size (at least if you are using the built-in Explorer; other apps such as JAM's TreeSize may show the correct amount).
Unusable for duplication: the amount of space that can't be used to duplicate your files, because of a combination of the different sizes of your pooled drives, the different sizes of your files in the pool and the space consumed by the "Other" stuff. DrivePool minimises this as best it can, based on the settings and priorities of your Balancers.
More in-depth explanations can also be found elsewhere in the forums and on the Covecube blog at http://blog.covecube.com/
Details about "Other" space, as well as the bar graphs for the drives, are discussed here: http://blog.covecube...2-0-0-230-beta/