[The Drummer]

Let me be clearer:
My System:
Mac Pro(Mid 2012)
2 x 3.46 GHz 6-Core Intel Xeon
64 GB 1333 MHz DDR3
NVIDIA GeForce GTX 970 4096 MB
Sierra OS 10.12.6

PT 12.8.2.106/2x 16x16 Avid HD interfaces

The Mac is used both for Audio(PT) and for Video (Adobe Creative Cloud Suite)
I have two separate System Drives(Audio & Video), both Samsung Evo 1TB SSDs
both running Mac OS 10.12.6, both in the Mac Pro’s internal trays.
OtherAttached Drives:
PCIe card with two 1TB Samsung Evo SSDs in a Raid 0 config as the Video files drive
Crucial 1TB SSD as a Video Cache drive and VI sample set drive(Video cache and Audio sample sets never accessed at the same time)
3rd Mac Pro internal tray: Samsung Evo 525GB as 2nd VI sample set drive.
4th Mac Pro internal tray: Hitachi 2TB 3.5” spinning disk for Audio Files and PT sessions only-
This is the most obvious culprit for PT access problems.
PCIe card - 4 port USB3 interface with 4x4TB Hitachi 3.5“ spinning disk drives connected which have Carbon Copy Clone backups taking place every night, backing up all the other drives. In theory, these drives are never accessed by active apps but they are there spinning all the time.
Just realised this is 11 drives!
Keep in mind that because of all these drives, when I tried to unlock the startup disk dialog to change system drives, I would wait a minute or two for the authorisation dialog to appear UNLESS I did it immediately following the boot up of the Mac Pro. I surmised this was because the drives had no time to spin down. If I worked for a few hours on an audio project and then tried to change I had to wait - probably while all the disks spun up. That put me onto a potential solution for these PT errors.

I also think that if people on MacBook Pros with soldered-in Apple SSDs are having problems, it's because Apple are doing some background tasks on the drive(Trimming?) when the drive thinks its idle(session cache in PT?) Again Avid would have to write a HD poker. But we know that AVID's recommendation is for a separate drive for audio files and that means an external drive for a laptop. Given I am now on Day 5 of seamless PT operation, the more all this makes a kind of sense to me.
I hope this helps
jonh