I feel like this should be a very common use case but the documentation around this is very unclear.

 

I currently have 4x 4TB drives, one being used as parity. They are all healthy. I've recently bought 2x 12TB drives that I want to add to the system, replacing the current parity with one of the new drives. I have enough bays in my server so I will eventually end up with 6x drives.

 

How do I add the two new drives to the system?

Least steps I can think of.

 

Set the array to not auto start in disk settings.

Safely power down, physically install the 2 new drives in the server.

Power up, go to tools, new config, preserve all.

Main page, rearrange disks to how you want them. Be VERY careful with BOTH parity slots, any drives assigned to parity1 or parity2 will be PERMANENTLY ERASED. Don't accidentally put a data disk in either parity slot.

Start the array, and let it build parity, optionally format the new data drive and old parity drive as well.

Once parity is built, do a non-correcting parity check to fully exercise the new drives.

 

All this assumes all your current drives are perfectly healthy and free of any errors. You can change the data drives to be in different data slots, or leave them how they are and just put the old 4TB and new 12TB in new slots at the end or totally rearrange the data slots, it doesn't matter. If your shares were included or excluded to certain drive slots those may need to be adjusted.

Is there an approach that copies the old parity drive for more safety and to avoid reading all data disks to rebuild parity?

You can do parity swap but you would have to add the other drive later and Unraid would have to clear it unless precleared.

3 hours ago, Tibbe said:

Is there an approach that copies the old parity drive for more safety and to avoid reading all data disks to rebuild parity?

not really.   There is the Parity Swap procedure but that only applies when simultaneously upgrading parity and replacing a failed data drive with the old parity drive, but since you want to keep all drives that does not apply.

 

If you use @JonathanM approach but avoid formatting any of the ‘unmountable’ drives until the parity build has finished the old parity dtive will be left unchanged to give you a fallback if something goes wrong.  Alternatively you could simply not assign the old parity disk to the array initially, and only assign it after the new parity is built but that would take longer as the old parity would now need go through a ‘clear’ operation to zeroize it when adding it to the array.