I think it says busy because it is trying the read and understand the zfs partitioning scheme and it is messed up in a nontrivial way. nfs-kernel-server is a support package for the NFS (network file system) an older, unix-based network file sharing protocol--before SAMBA and Windows sharing.
tgalati4@Dell600m-mint14 ~ $ apt-cache depends zfs-fuse
It's not a required package, just a suggested one, probably to allow you to share a zfs pool over NFS. So removing that package should not effect the operation of zfs pools.
So to summarize:
1. You built and operated a 12 TB zfs pool and moved data around at it seemed to function OK for 2 weeks. Hardware is probably OK.
2. You were finished playing around with zfs so you decided to use the same drives for software RAID pool using mdadm. You repartitioned them (wiped them) and created ext4 partitions and formatted them with mkfs.ext4. You used parted, which I believe has no support for zfs (at least gparted doesn't have support in 12.10). So there may be remnants of the zfs pool striping on the drives that zfs can read, but parted did not erase.
3. ZFS detects the striping of the drives, or a configuration file and is trying to read them resulting in busy. Mdadm shows the current RAID with ext4.
I'm not familiar with the zfs-fuse package, but from the name, I assume that this is a user-space zfs mount tool that allows users to make and mount zfs pools. Because it is user-space and not kernel-space, there are probably configuration files stored somewhere, like ~/.zfs perhaps? Try finding and deleting those configuration files. There might be files in /etc/zfs.
Try removing the zfs-fuse package. If you are not going to use zfs, then remove it. This assumes that you have no other zfs pools that you need access to.
The second one removes configuration files as well as removes the package.
sudo apt-get remove zfs-fuse
sudo apt-get purge zfs-fuse