Long story, I'm afraid you'll have to wait for the punch line.
I finally made the leap to Yosemite from Snow Leopard. 10.6.8 was the most reliable OS I have experienced, everything worked, nothing crashed and I was extremely reluctant to move from it.
I kept a drive fully up-to-date, however, just to see what the new OS's looked like, read the tech reports, etc, etc.
Finally, I had to upgrade. The modern browsers need the latest OS, many websites I use say my browsers are out of date and I've been trying to find a reliable solution to bringing USB3 to my 2010 Mac Pro.
The transition has been relatively painless. The OS works just as well, but I have to learn the new wrinkles, notifications, eye candy and whatnot. A couple weeks of tweaking.
The Sonnet Allegro USB 3 pci-e card was plug'n'play and I quickly filled up the four ports. My external Toshiba HD fills up at 9.5 Gb/minute now. The hard drive caddy mounts all my old
SATA drives @ USB 3 speeds, too. All my browsers have stopped complaining. All a small price to pay for losing GoLive.
My DreamWeaver CS3 Missing Manual arrived today. It cost £0.01 + postage. Hours of fun.
All roses until I started to get a pop-up warning: USB Devices Disabled - Unplug the device using too much power to re-enable USB devices.
Everything worked. All devices were mounting and working. The warning seemed spurious, yet I was getting it every minute or so, even when no device was drawing current.
Much internet searching ensued. The cheapest solution to try was to reset SMU and PRAM. Done. The next cheapest was the possibility that the PRAM battery was failing in my 5-year-old Mac. So, I got online and sourced a battery. It arrived today.
Okay, the punch line:
Since resetting the PRAM, I have not had a repeat of the USB warning. So, I took my new battery in its cardboard holder and pinned it up on my bulletin board next to the computer (wait for it) RIGHT NEXT TO the two other identical batteries that have been pinned there for who knows how long.
Mac Pro dual-2.4 GHz, 10.13.6, 24 Gb RAM, 250 Gb Samsung EVO SSD/Velocity Solo PCIe card, 2x3Tb Seagate HD, 1x3Tb Hitachi HD, Dell 2408WFP; Canon PIXMA iX6550; CanoScan 8800F; MacBook Air 1.8 Ghz, 8 Gb RAM, 10.14.6, 256 Gb SSD; Vodafone Home Hub/BT Wi-Fi Extender.