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"I am pretty sure that when I was researching this last week I could claim the disks for both tiers."
And what did you change since then?
If Deduplication is enabled on the cluster, please confirm that you have valid, non-expired vSAN Advanced or higher license (and that you have a valid license in general otherwise).
I saw issues such as this frequently with 6.7 U2 hosts but 6.7 GA/U1 vCenter so confirm that you are using a 6.7 U2 or later vCenter first and all hosts on same build.
Another issue we see perennially (and frequently after major updates) is custom-certs that are not entirely functional - one can confirm whether the issue is host or vCenter side by validating whether you can create Disk-Groups via the CLI:
# esxcli vsan storage add -s naa.xxxxxx -d naa.xxxxxxx
So the servers did have ESXi670-201908201-UG and ESXIi670-Update03 applied early today... vSphere Client version 220.127.116.11000
We have a valid license (going to check it) and this is the first attempt at building vsan so no previous config.
Actually vSphere Client version 18.104.22.168000 comes with vCenter 6.7 U2:
Upgrade your vCenter to 6.7 U3 as this is the only version that supports vSAN hosts running 6.7 U3:
You can of course validate that the issue is vCenter-related by temporarily configuring a Disk-Group via CLI using the command noted in my previous comment.
Upgrading to vCenter to 6.7 U3 solved the issue and I can now select flash drives as capacity.
Many thanks for the help.