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Enabling duplication for your entire pool will default to doubling the used space of your pool. So yes, if you had 36TB of unduplicated files in the pool, enabling the default duplication (x2) would result in those files using 72TB of space (assuming you had that much space available). To answer a later question in your post: if for example you had drives A, B, C and D and you had a file on B (inside the PoolPart folder), enabling duplication x2 will put that file also on either A or C or D. Adding a drive to a pool will NOT put any pre-existing data on that drive into the pool (which is why the pool still looks empty when you add a used drive to it; it's only making use of the free space of that drive, it's not automatically adding files from it). Any pre-existing data will NOT be touched, remaining where it is. You have to move that data yourself if you want it in the pool. It's fine to move pre-existing data on a drive directly into that drive's hidden PoolPart folder to get it quickly into the pool; you just have to be careful to avoid accidentally overlapping anything already in the pool (e.g. on other drives) that has the same folder/file naming. To avoid that accidentally happening, I suggest creating a unique folder (e.g. maybe use that drive's serial number as the folder name) inside each PoolPart and moving your data into that if you're worried; once you've done that for all the pre-used drives you're adding to the pool you can then move them where you want in the pool normally. If you're moving data directly into the hidden PoolPart folder, after you've finished you should also tell DrivePool to re-measure that pool (in the GUI, click Manage Pool -> Re-measure...) so that it can "see" how much data is on each drive in the pool. This helps it perform any balancing accurately. If you use DrivePool's "Remove" function to remove a drive from the pool, the data inside that drive's PoolPart folder will be moved from that drive to other drives in the Pool as part of the operation. Any data on that drive that is outside of that drive's PoolPart folder will NOT be touched (because that data wasn't in the pool). DrivePool doesn't keep a record of which files are on which drives in the pool. You would need to keep your own records for that. Regarding using SnapRAID with DrivePool, #1 you should turn off (or be very careful with) DrivePool's balancing, #2 if you're only going to be using the drives for DrivePool with no data outside the poolparts then I suggest using the PoolPart folders as SnapRAID's mount points as per this post. Your mileage may vary however.

So in case anyone else comes here I have reduced it down to 628gb of "Other" on 110tb of disk. I've determined (using a blank disk) that the root cause is the checksums for ReFS metadata. See https://learn.microsoft.com/en-us/windows-server/storage/refs/refs-overview I can reduce it using NTFS, but I prefer ReFS for integrity. I know that is a controversial topic but I've made my choice, I'm happy with it and over many years of using ReFS with DriveBender I haven't had a problem. Big thanks to Shane and Christopher for offering suggestions.