I have been running FCPX version 1.4 successful on my IMac 14,2 / Intel Core 17, 3.5GHz. I have both a G Tech Thunderbolt 4 Terabyte and 3 terabyte. hard drive connected via thunderbolt cable.. To use FCPX 2.1 I had to upgrade my OS X to Yosemite and then download upgrade FCPX 2.1. Since then Final Cut takes forever to load completely onto my computer (up to 10 minutes) stutter steps during playback, drops frames, and shows me a color wheel everytime I make a cut or add to the time line. I spent 3 hours on the phone with a senior tech at Apple to no avail. Any answers to this problem would be greatly appreciated. Anyone having similar problems I would love to hear about it. Evidently going back to the old OS and FCPX 1.4 not an option. Thanks.