Your new Mac Pro is formatted APFS. It can see non-APFS drives but non-APFS drives cannot see your Mac Pro. So no surprises that you can't run BIAB entirely from an external mechanical hard drive (HHDs can run High Sierra but cannot be formatted APFS).

The 2017 installer does work over High Sierra so the 2016 should also. You'll find it on the web site
PG Music > Home> My Account> My Products

You'll need to install Band-in-a-Box 2016 Full (Mac) and MIDI & Audio Plugins (Mac). The rest can stay on the drive which you will need to link to BIAB (pretty easy) or you can install everything onto the Mac Pro. There was a patch for 2017 to fix minor High Sierra issues but I don't know about BIAB 2016.

The only thing that can't be downloaded is Audiophile RealTracks if you have that version. Those can be read from the hard drive or loaded onto your internal (if you have the space) or an external SSD for reduced latency. SSDs and USB 3 docks are relatively inexpensive these days.

I have the minimum installed on my System drive and everything else is loaded onto an external SSD.


BIAB 2024 Audiophile Mac
24Core/60CoreGPU M2 MacStudioUltra/8TB/192GB Sequoia, M1 MBAir, 2012 MBP
Digital Performer11, LogicPro, Finale27/Dorico/Encore/SmartScorePro64/Notion6 /Overture5