Questions about Duplication. in General Posted May 10, 2020 So AFAIK, everything DP reports wrt space is in terms of actual disk space. Assume a Pool of 2 x 10TB drives, x2 duplication except for one folder named UDF (UnDuplicated Files). - Pool is empty and shows 20TB free - Save a 1TB file to the Pool. The Pool will show 2TB Duplicated and 18TB free. - Save 2 100GB files to UDF (two files because one would complicate a bit and cause Unusable for duplication, now one is on one drive, the other on the other). Pool will show 2TB duplication, 0.2TB unduplicated and 17.8TB free. A duplicated file is indeed seen as a single file _assuming_ you access the file through the Pool / DP. However, a x2 duplicated file is stored on two HDDs. Each has a hidden PoolPart.* folder within which the file resides. If you access the HDDs directly, you can see two files. In the above example, if you pulled one HDD (so it is still working) and put it into another PC, then yes, on that PC you can simply navigate to the PoolPart.* folder and find all your, duplicated, files (unduplicated is a matter of chance of course). DP stores everything in plain NTFS so that is nice from a recovery point of view. Now assume one of the 10TB drives crashes, hard. DP will put the Pool in read-only mode. You will add a new 10TB HDD to the Pool and remove (through the GUI) the faulty one. At that time, DP will check for duplication and find none are duplicated even though most should be. DP will then re-duplicate. Non-duplicated files stored on the faulty HDD are lost (barring recovery and/or actual backups). I guess that's basically it.