I just updated my iMac 2009 to Yosemite and updated to Xcode 6.4 (I haven't used this computer for ~1 year). Everything seems to work fine when i build to the simulator, but if I plug in my iPhone or iPad and try to build to the device I immediately get an error that says "Xcode encountered an error. Could not allocate a resource."
I tried the suggestions here, and a bunch of variations on it, but nothing fixed it for me: Xcode error when running on device: "Could not allocate a resource"
Update:
I'm using an iPhone 5 on iOS 8.4 and iPad Air on 8.3. The alert is a pop-down thing in Xcode. Here's a screenshot.
I should add that running the same code+same version of Xcode+same devices on my laptop (rMBP 2013) works fine