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Hi there. Sorry you’re having troubles.
Just to say, “you’re not crazy,” I have had permissions issues as well, just attaching an external volume with DVR content created on a MacOS PMS to an Ubuntu OS, PMS-combo, booted from a different drive on the same MacPro hardware. I wanted to see if the Ubuntu OS/ PMS combo on MacPro hardware would be any faster than MacOS 10.10.5 and PMS—it’s not by a long shot—the MacPro is a 1,1 box is patched-to-hell getting beyond 10.7!
Best of luck finding a solution… I will be lurking to see what you learn.
Cheers!

News flash!
This might be helpful:
‘Forum Tips’ on Linux drives migrating to other OS