4tb + 2x2tb disks - can I create these mirrors?

Sorry no. You cannot mix RAID on the same volume and in a RAID volume, all partitions are the same size.

You can create two mirror volumes though.

Initialize all 3 disks with SoftRAID.
Create a Mirror volume with the 4TB and a 2TB drive
Create a second Mirror volume with the 4TB and the other 2TB drive.

I would select the 4TB as the primary for both volumes.

>> Create a second Mirror volume with the 4TB and the other 2TB drive.

This is the step I'm unclear about. After mirroring a 2TB and the 4TB, which will be limited to a matching 2TB, what are the steps to 'select' the second 2TB on the 4TB drive as the mirror?

When you create the second volume, the 4TB disk will also show up in the selection window. Try it.

When you create the second volume, the 4TB disk will also show up in the selection window. Try it.

Thanks. I have some files to move around first but then will try the above.

Okay, Just a couple of hours away from completing the second 2TB mirror's 'rebuild'. Thanks.

BTW, a reminder to those building or populating file systems: Turn off Spotlight (in System Preferences) on that file system or it will consume a lot of time reading/indexing the file system when you'd rather having it writing. You may also want to check Time Machine's settings to see that it isn't going to try to back up your new filesystem, too.

Just to follow up... Builds went fine. Now have two 2-TB drives mirrored onto a single 4-TB drive. Looking good.

Previously I'd copies various photo and video folder/libraries kinda willy-nilly between drives. Now I have confidence that I have at least two copies of everything and they take care of updating themselves.