I currently have 33TB of media (48TB total) on my Unraid server and it is working great. Backups have been tedious across the LAN to my Windows client using multiple USB drives.
I have now set up a Terramaster D4-320 with 3 -14TB drives. AI suggested that I configure them as one drive using RAID 0 to get the optimal backup speed through striping. I'm not too concerned about a higher level of RAID because that's already set up in my primary Unraid storage.
Now, I've learned that Unraid will not suuport a software RAID 0 on my Terramaster DAS.
What is the best way to set up these drives as a single unit for backup purposes, prioritizing backup speed?

Speed isn't really much of concern for backup since once its done it only needs updates instead of whole backup.

I would just set it up as an "ordinary" Unraid array, then after initial backup taken you can add and build parity if you want.

I guess you're right. I was thinking speed because I was comparing it to what I've been dealing with over the LAN.
Now, I've got to figure out how to set it up as a new "ordinary" array. I set up my Unraid years ago and I've been pretty good at updating it and expanding it. Now, I'm I'll try to create a new volume. I've already erased the data on the drives that I put in the DAS because they were the drives that I had originally in my Unraid and I just upgraded those drives to larger ones. Now, those drives are just presenting themselves as 'Unassigned Devices' asking me to 'FORMAT' them. I'll be back here if I need more help.
I probably could have tried a NAS instead of a DAS so that the array would be running on it's own.

Sorry, I assumed your backup would be running Unraid.

I'm not sure what you mean by that "I assumed your backup would be running Unraid"
Can Unraid keep to arrays going or do I have to add these three new drives that are in the DAS to the existing array?
CoPilot is telling me that I can manage the DAS drives outside the array and pool them with MergerFS or something similar for one large backup volume which would avoid mixing parity protected disks with non-parity protected disks.
What do most people do to manage their backup volumes?

Usually backups would be on a separate server in case something catastrophic happened to the original server.

Unraid doesn't support multiple arrays (yet), but you can put them in a multidisk btrfs or zfs pool outside the array. Different btrfs or zfs configurations could provide redundancy or speed or both.

https://docs.unraid.net/unraid-os/using-unraid-to/manage-storage/cache-pools/