Creating a Softraid mirror with primary disk using partition on internal SSD, secondary disk using partition on external SSD?

No, because SoftRAID volumes cannot boot, so you cannot have the SoftRAID partition maps on a startup disk, i.e, your "spare" 1TB volume.

Best might be set up a second volume as you want, but use Carbon Copy Cloner (or equivalent) to keep it up to date.

I know that booting from a RAID was removed from SoftRAID since v6, after macOS 10.15 Catalina was released back in Sep 2019, because Apple no longer supports this. Are you saying that the ability to even create a non-RAID volume with SoftRAID to use as a boot drive was also removed? If so, was this removed at the same time? Also why, as here is one good use case, which does not involve a boot volume on a RAID. And the data partition would not be used for booting?

@smayer97 

A disk can only be controlled by one driver. If a disk is bootable, it must be controlled by the standard MacOS disk driver. Therefore, SoftRAID cannot control a "partition" on that disk. that is why. You cannot have two disk drivers controlling the same disks.

@softraid-support Thanks. I understand that you cannot have 2 drivers. What I am asking is why not allow SoftRAID driver on a boot drive, as non-RAID partitions? This way the data partition could then be used as the OP is wanting.

Are you saying that Apple does not support any other driver (AFAIK, there is only SoftRAID now available) to be on the boot drive? This is a different statement than Apple not allowing booting from a RAID.

@smayer97 

Drivers are not on disks. I think that is your misconception. Drivers are in the system itself.

What happens at startup, is MacOS looks at each disk's partition map and asks "who owns this". This is disk based, not partition based.

For SoftRAID to control a disk, the disk has to be owned by SoftRAID (have a softRAID partition map)

Only the standard disk utility formatted disk can startup a Mac now. You cannot have different drivers controlling different partitions. not possible.

@softraid-support Thanks. I'm not asking to control different partitions by different drivers. What I am asking about is controlling the entire internal drive, as non-RAID volumes/partitions...does Apple no longer allow SoftRAID to control a non-RAID disk that has macOS installed?

Help me understand, because IIRC, all announcements about macOS and RAIDs was that Apple no longer supports/allows installing macOS on a RAID, and hence booting from a RAID. But what about non-RAID volumes created by SoftRAID? Does Apple allow the installation of macOS on a non-RAID volume created by SoftRAID?

If yes, then why was that feature removed from SoftRAID; is it because there is a problem with macOS being on a non-RAID volume created by SoftRAID?

@smayer97 

This is Apple. they started preventing installing MacOS on SoftRAID volumes in 10.13. But you could work around it, or clone to it.
no longer. You cannot install onto a SoftRAID (or Apple RAID) volume, nor can you clone to one and be bootable.

Its not us. Its Apple security measures that have gotten more extreme over time. Just like you can no longer boot from an HFS volume any longer.

Thanks for the helpful info.  

Just curious, if I have an external SSD, could I create two partitions and use one partition as a Time Machine backup destination, and the other partition as a mirror to another external disk?    

@softraid-support OK thx.... bu you seem to only be talking about the driver and/or the software.... so to be clear on the language, I am asking about the volume format, that is a SoftRAID non-RAID volumes... Could macOS boot off a non-RAID volume created by SoftRAID?

No, a SoftRAID non RAID volume still has the SoftRAID partition map, and MacOS does not allow anything but a standard macOS partition map to be bootable.

Yes if you create a Time Machine volume (has to be APFS) as one volume on a disk, the rest of the disk can be used as a Mirror volume with another disk.

the only limitation is boot volumes/disks.

@softraid-support Thanks for the clarification.