Great info, thanks.
But is 6GB a typo? You mean 16GB? If 6GB we really need to talk.
So there is a lot going on here and I'm going to try to get you pointed in the right direction on some stuff and map out some ideas to look at.
We throw around standard troubleshooting or optimization stuff on DUC... all that basic stuff is described under the "Help Us Help You" link up top of every DUC web page. The optimization instructions are also in the Pro Tools Installation Guide. Take your time and read all that stuff carefully.
While troubleshooting do the full systems optimization. Just get it all done, every last damn annoying thing and leaver them all done even if they don't show an immediate improvement why you try other things. That means disable WiFi, Bluetooth (try a wired keyboard and mouse) etc. Just plow though doing all those and then try out Pro Tools.
Trashing prefs is described on the troubleshooting notes but make it easier for yourself and download and install Pete Gates's tool and just use that.
http://petegates.com/pg-pt-prefs.html Just about any troubleshooting with Pro Tools should start with trashing prefs, and maybe doing it multiple times as you try different troubleshooting things out.
When troubleshooting CPU errors make sure to try the following (as well as other things)....
0. Always start with trashing prefs.
1. Make sure ignore errors is not enabled (you want pro Tools to tell you about problems).
2. Disable "dynamic plugin processing" and leave it off forever. It really does not help and only causes problems.
3. Test starting with a new empty session, not a copy, not from a template.
4. Move all plugins out of the plugin folder, Start Pro Tools it will magically put back just the standard plugins, test with those. (Many CPU problems are caused by plugins... *always* suspect plugins. They can cause problems just by being installed, not even used in your session).
6. Test with built-in output as the playback engine (that test if your interface driver is buggy, some are)
7. Create a new admin account and test from there... with a new empty session, plugins removed, prefs trashed etc.
8. Look out for denormalization issues with certain plugins (like DVerb, Sansamp) that can cause run away or spikey CPU use. Or you see the CPU meters run away when the transport is stopped. Only happens if the plugins are in a sessions with the plugins getting very low signal levels, remove those plugins to test, or stick a dither plugin in front of them.
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It would still be useful to know what exact Porsche Design drive you have. You should look at the model number/find it online and try to verify what the RPM specs are. If you can't find a statement that it's 7,200 RPM then assume it's 4800 RPM as Christopher mentioned. In which case you should not be using it anyhow for an audio drive. Disk cache can hide lots of sins but you should still not be using a 4,800 RPM drive.
Your Mac has a Thunderbolt port, a FireWire 800 port and multiple USB 2.0 ports. USB 2.0 is pretty slow. The likely best option is to just install two SATA SSD inside the Mac Mini. One for OS, one for an audio/session drive. If you have samples put them on the OS drive. Adding a Samsung 860 Evo would be my choice.
If you really had to go external look for a Thunderbolt 1 compatible external SSD, this one is an option...
https://www.amazon.com/LaCie-Rugged-.../dp/B071FWB8L2
Oh my bad before... Yosemite was not a huge problem release, that was El Capitan (El Crapitan). But if you try other troubleshooting and it does not work then try a full clean new OS install. And if you are getting another internal SSD drive for sessions I would take that opportunity to install a new clean macOS image on it, install just the bare minim you need, like ILM and Pro Tools and test that with Build-In output, no third party plugin installed, nothing extra. You can put the sessions on the other SSD you have now while you test with the system booted from that new install. If that works fine you can install everything else you need. Do not use system migration utility to move application or drivers across macOS install, install all software fresh using the correct installers.
You need to check and work out what the latest macOS version is that your computer supports and what versions of Pro Tools is compatible with that.... OK I'll do that for you...
https://everymac.com/systems/apple/m...ver-specs.html Looks like High Sierra, which was a nice release for me on my MBP, and that gets you official support for Pro tools 2018.10. And High Sierra gets you APFS filesystems support, which I'd use for the new SSDs and maybe upgrade the old one to even though it's not officially supported.
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When troubleshooting stuff try to be systematic, take notes or work form a printed list checking off stuff as you go.
If you need more help let us know exactly what troubleshooting you have done, and exactly what error messages you are getting.