Yosemite 10.10.1 upgrade to External SSD...no problems
I run a slightly different SSD setup than most users. I have a mid-2011 2.5GHZ I-5 Mac Mini booting from a Seagate Thunderbolt sled with a Crucial MX100 256GB SSD. I'm also running Trim Enabler.
I was a little nervous upgrading to 10.10.1 but I just followed the recommended sequence mentioned previously and the installation was flawless...albeit with a few extra steps to disable and enable Trim. Trim Enabler is doing its thing according to System Report.
I keep a full copy of the SSD on the old 5400 rpm internal HDD and upgraded that, too, while everything was in flux. The Mini is very quick with the SSD and Yosemite, and I'm a happy camper.
Rich
Rochester, NY