*ugh* placed a big order for hard drives, which is painful but half was due anyway:
* 4-bay expansion for the NAS
* 4x 16TB drives for the new dedicated media library array
* that frees up the 2x 16TB hot spare drives I used to make the temporary array to be spares again, to be available for both primary and media library arrays
Thus I'll be able to survive two separate single-drive failure events without further purchases. Should be good insurance!
Undecided between RAID 10 or two RAID 1s.