You would plan it that way because a) the internal SSDs provided by Apple are far too expensive,
Let's look at that: A 1T X5 is $500 and a 2T is a grand (round numbers). That's $100 & $200 less than adding when the Mini is
Built
To
Order. I have no problem with those prices to keep projects on the boot drive and reduce clutter.
Now look at the differences in the Refurb Store where there happened to be a few this morning. Both RAM and SSDs are less expensive than adding aftermarket and externals. 1T and 2T when you find them were $400 for 1T & $800 for 2T. There happened to be a 1T version of the one I linked ant it was $400 less. Both are gone this evening. That refurb I pointed to was $504 less than BTO.
I found my iMac Pro used at a bargain. The difference between 1T and 2T is $600 ($400 refurb) but the difference between 2T and 4T is another $1,800 and I couldn't justify that. I need 2TB on board and can get by with everything else over USB 3. With SATA III drives, the SSDs are the bottleneck—there's no difference between USB and USB 3.1 over USB-C. I timed that last week. Anyway, by springing for 2TB onboard, I have no need for an TB3 external such as the X5.
all the current project on another external SSD
Yea... that's a bad idea anymore. Ok, it's not been a good idea since OS 10.4 but, with APFS Snapshots, it's really a bad idea nowadays.
DP is the only app where I've used Snapshots to get out of trouble—I've posted those details before. It couldn't have happened had my projects been on another drive.
Now I do testing and sometimes I just screw around. Just yesterday, I tried to make DiscLabel work in Mojave so I wouldn't have to learn
Mac CD DVD Label Maker. Did some deep tweaking based on crash reports and totally hosed my system—details aren't necessary but, without Snapshots, I never would have tried it. I timed it: A complete restore took 92 seconds from the time I threw in the towel till I was staring at my system 5 minutes before I totally screwed up.
... and, the tests I looked at when the Mini first came out had the external SSD "as fast or faster" than the internal.
It depends on the tests and applications. Possibly the smaller ones that are single blades but not the 1T and 2T which are twin blades with a separate NAND controller. These scream.
All the sample libs are on an external SSD
Yea — a USB 3 drive is certainly good for that. My iTunes and Photos are offloaded too.
and a load of USB 3 drives for backup, pictures, music, and seldom accessed libs of other stuff.
Ok, but for someone looking for a new system, why would you recommend that? Now that SATA III drives are under $230 for 2TB. Then you have docks and hubs and... I'm in the same boat and need another drive. Wish I could find that 4TB 860 QVO for less than $500 but I might buy one anyway to just to reduce the clutter.
Speaking of clutter, I have a twin drive TB2 dock on the way so that I can put all of my VIs and non-current projects onto one piece of hardware and connect through the TB2–TB3 adapter. Besides seeing both drives in a single dock, the one I'm getting has a second TB port. I'll try running my 828mkII through the FW–TB adapter to it. I should be able to get the 828 and 2 large SSD showing up in one TB3 port.
I'll still load test drives into the USB 3 toaster but I don't want more than two docks up there.