TLDR: I need to promote an 18TB array drive as a 2nd parity, and replace it on the array with a smaller 14TB drive.
Short story long...
I purchased 2x 18TB drives last year. I did the "parity swap" dance with a 12TB drive.
Instead of being actually smart and adding the other 18TB drive as a 2nd parity right then and there, I didn't. (to be fair, I was waiting on a HDD caddy for more mounting space)
Fast forward to one of my 6TB array drives failing a few months later, so I panic-swapped it to my only spare drive, the other 18TB, so my kids could continue to watch their precious Coco on repeat on Plex.

I picked up 2x 14TB drives during the holiday sales.
I'd like to now replace the 18TB array drive with one of the new 14TB drives, so I can utilize the 18TB as a secondary parity drive. However, I find myself a little stuck not being able to replace the 18TB array drive with anything smaller (understandable)
From my understanding, the failed drive or disk swap parity dance doesn't really apply since I would be reducing the total size of the array, but surely there's a "safe" way to accomplish this? TIA!

The only way I can think of is:

• Use Tools->New Config (with Preserve all assignment)
• Return to the Main tab and put the 14TB in place of the 18TB.   Leave the 18TB unassigned at this point
• Start the array to build new parity based on the 14TB drive being in the array
• Format the new 14TB drive to get it ready for use
• Mount the old 18TB drive via UD, and copy its contents back to the array.
• Once all the data has been copied off the 18TB drive it is now ready to be assigned as parity2

I found New Config referenced a few times but more related to disaster recovery, so I shied away. I will definitely try this out!

There is a good write up on New Config here in the online documentation accessible via the ‘Manual’ link at the bottom of the GUI or the DOCS link at the top of each forum page.