If you show the package contents of LaCie RAID Manager in your applications directory and manually run the bash script to install the driver (InstallDriver.sh) in Terminal you'll likely get a message saying it's only x86 compatible. At least, that's the case with my big 5 bay LaCie. After 3.5 hours on the phone with support this is being blamed on Marvell and LaCie says they can't support it and don't know that Marvell will choose to on 'legacy' devices like this. I got a message (similar to the usual kext security messages) on install informing me of the incompatibility (when using the regular install GUI). It's fairly easy to miss and you'll only get it on the first install. Did you see anything like that?
I'm not sure if the root cause is the same on your TB3 6-bay but this is the dead end I hit on my TB2 LaCie. If they're both using the Marvell controller it seems likely.
Pyriphlegethon
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