Yosemite killed FCPX 10.1.3

Question

Since upgrading my iMac (24-inch, Early 2009, 4GB) to Yosemite (final release), Final Cut Pro (10.1.3) no longer works. I get the following error: "This graphics configuration does not support hardware acceleration, which is required to run Final Cut Pro." I have tried deleting my copy of FCP and redownloading from the Apple App Store, but still nothing.
It has been working fine under Mavericks. I know it's my fault for not researching Yosemite compatibility with this iMac, but I need a fix. I either need FCP to work under Yosemite, or I need to know how to downgrade back to Mavericks. Thanks in advance for your help.
iMac (24-inch, Early 2009)
3.06 GHz Intel Core 2 Duo
4 GB 1067 MHz DDR3
ATI Radeon HD 4850 512 MB

Answer 1

I have a similar Mac (still on Mavericks), and even though I have only occasionally used with FCP X (as I use mostly my 2010 MBP and now my rMBP), yours is a cautionary tale for me.
It may well be that the GPU is borderline for this kind of work and that with Yosemite it does not work anymore.
No idea on how to make it work with Yosemite.
Note: of course most macs that worked well with FCP X in Mavericks continue to do so in Yosemite. The following may apply, however, if an old-ish mac used to work with FCP X in Mavericks, but now does not in Yosemite.
Going back to Mavericks should be possible, but is not trivial.
0) Open the Mac App Store and find OS X Mavericks in your Purchases. You should be able to download it.
I don't think you can just reinstall Mavericks over Yosemite, though. You may have to reformat the drive, which also means that you probably should:
1) Clone the drive to an external (since you are going to need to reformat it)
2) From the a Mavericks installer create an installation disk on a pen drive or similar (instructions for this abound on the web - search for "Mavericks createinstallmedia")
3) Start up holding the option key, select the aforementioned pen drive, erase the iMac drive, install
4) Copy your stuff back - at this point it is not clear but I expect that Migration assistant should work to import your stuff correctly.