Usually I just leave DrivePool to do its thing, but tonight I checked in with its control panel on my WHS2011 home server for the first time in ages, and was confused (and somewhat alarmed) to see 101GB of unduplicated files. I use file duplication, with a fairly anal set of rules to force certain folders onto certain pairs of disks (Docs & Pics are duplicated always on disks 1 & 2; Music & Videos always on disks 3 & 4). This is so I can mix HDD brands for added protection.
I was utterly baffled by what seemed like an 'undupe-able' 101GB of files, and tried rebalancing, remeasuring, checking the rules etc. I logged in here to ask a question & post some screen shots.
Fortunately before I had time to make a prat of myself I noticed something I'd missed in the 'file placement' tab: I'd been so busy checking & rechecking my folder rules, that I'd totally overlooked the 'Recycle Bin' line. My Recycle Bin isn't duplicated in the same way that my 'real' folders are (why bother?).
Checked the Bin's properties - it contained exactly 101GB of stuff I'd thrown away. Emptied the Bin, and all my DrivePool stats reverted to normal (equal amount of files on the appropriate disks, everything duped). The moral of this story: if you have inexplicable duping stats weirdness, check the Bin, and empty it. I'm posting this as confession is good for the soul; as potential help for others; but mainly so that when I've forgotten it in a year's time and I have the same problem again, I might find the answer on this forum from my past self...