Increasing RAID size

You almost have it, and I think you would have figured it out.

I would validate the volume first, just to make sure all the parity data is up to date and be sure to have a backup (never forget RAID is a strategy, not a backup!)

Yes, use the "remove disk" command, then "add disk" with each disk, one at a time.

When both are replaced, there is one more step:
"resize volume". select the max volume size.
this will allow you to get the additional 1.6TB capacity for your volume.

Ok cool, make sense. Another question, this one more general. A RAID as a strategy is to gain a high capacity disk while guarding against loss of data due to one drive suddenly failing. The problem with backing up 9 TB of data is you need of course another 9 TB of storage in some way. What are the dangers of making the backup drive yet another RAID aray?

Better than nothing! We have heard reports, for example, of a rogue application from Western Digital suddenly initializing ALL disks connected to a system. In that kind of event you would not be covered. Also, the kind of corruption caused by data hijackers, who encrypt your data and ask for ransom. (more common in the Windows world, but no reason to be too complacent. A second array is very good, however. Backups are important, as often there is no recourse to data loss, or data corruption. So would an emergency alternative of cloud storage, which costs monthly, but hopefully you never need it!

Backups are important, as often there is no recourse to data loss, or data corruption. So would an emergency alternative of cloud storage, which costs monthly, but hopefully you never need it!

This is exactly what i intend to do. A similar RAID 5 setup to yours and then an online backup to BackBlaze as a starting point. And then a weekly archive/backup of the most recent work onto a set of external drives.

The only thing i still can't get over is the time it takes to certify disks. But god it feels good when all your drives passed!