When I upgraded from 10.6.8 to 10.10.3 this summer, it took a while to get most things working...
My new Seagate 2 Tb external hard drive failed. I sent it back to Seagate California from Canada. Within 2 weeks I had a new one, no questions asked. That's been working fine. They sent me a Customer Survey for feedback. I have to praise their service.
I had several font glitches from my 1200 fonts library. I used Kurt Lang's font management advice to solve that finally. He recommended to switch from Font Agent Pro6 to trying Suitcase Fusion. I still have to try that.
However, my Apple email cursor is mostly in the wrong place which is VERY frustrating. It's usually 4-9 spaces ahead or behind where it should be. That's a pain in the butt to type a letter or report and not know where to place the cursor. Takes 3-4 attempts each time to get it where it should be. I've googled the problem and tried many remedies in Mail preferences, using only system fonts, etc. Nothing solves this permanently. A few times the cursor stays in the correct spot for a while after rebooting and then it's back to being out of synch again. Aaargh!!
Any other good advice?
24" iMac early 2009. 2.66 GHz Intel Core 2 Duo. 8 gb RAM.
OSX 10.10.3
Only one user.
Apple Mail 8.2
1 Ext HD for Time Machine.
1 Ext HD for Super Duper.