Hello!
Just moved away from my Synology NAS after a bad experience trying to get files off it when the hardware failed - total pita!
I've got the following drives in a pool with folder duplication set on most folders:
8 Tb
2 x 4Tb
2 x 3Tb
..12.8Tb currently free, duplication free, nice green bar. It's not balancing any of the files to one of the 3Tb and one of the 4Tb drives though. Is this normal? Is it just not bothering as there's so much free space elsewhere?
Were all of the drives empty and added to the Pool before files were placed on it? Perhaps some of the files were manually copied into the hidden Poolpart-xxxxx folders in the root of each? Hard to tell without a screenshot and knowing how you populated the pool.
What you can do to even things out, is install the Disk Space Equalizer plugin for Drivepool and force a manual re-balance. You go into "Manage Pool", then Balancing, then enable it in the Balancers tab. Hit Save after that,