I'm in the exact situation here, right now my jails are relocated and now running into separate usb flash drive, and all my data (Movies, Pictures, Videos, etc.) are stored in here ->> (/mnt/tank/mymedia) 4x WD red RAID 5. I did this because I was hoping the HDD to go to standby mode by separating the Jails from the pool but unfortunately it didn't work.
So, I went ahead and purchased some desktop 4x HDD as replacement for my "NEVER go to sleep mode enterprise HDD" (Western Digital Red NAS Hard Drive).
Now, how can I proceed on migrating all my data from WD red HDD to the new desktop HDD without losing any single data or file.