I have just updated my OS to Yosemite, but now PS CS5.1 won't open. Any suggestions?

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The Yosemite update wrecked your Ps CS5.1 installation.  Thank Apple for that.
The workaround is to uninstall CS5, look for, download and run the Adobe CS Cleaner Tool, Repair Permissions with Apple's Disk Utility, install CS5 from scratch, apply all available updates to it, and then Repair Permissions with Apple's Disk Utility once again, in that precise order.
You should know that there are bunches of bugs and issues that Apple still has not managed to fix in Yosemite.  Users have been advised to downgrade to a more stable issue of OS X and some have.

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