On further analysis, I can't figure out why this pool is reporting as full to the OS. To get out of the woods, I simply added a 4th drive to the pool with plenty of space. I'm still not able to copy data to the pool drive.
Pool is now made up of 4 drives configured to duplicate 3x. One of the pool drives is full.
Since adding the 4th, nothing has rebalanced to it (I would have expected files to be moved from the full drive to the new drive).
Windows explorer reports the pool drive as having 3.63 TB free.
OS will not allow writes (larger than about 300Mb) to the pool drive saying that there's not enough space.
Any thoughts of what's going on with this pool?