I recently updated to Xcode 4.2. I also updated my new iPad 2 and iPod (latest generation) to iOS 5. I build my app and can debug them no problem.
My older iPod running iOS 4.2.1 (8C148) doesn't run and doesn't give any errors. The top center panel says "Finished running MyApp on iPod One". (iPod One is the name of my first iPod touch)
There is a flicker at the bottom of Xcode as if it almost went into the debugger and was about to show the bottom output panel but it doesn't. A break point in main doesn't stop it either.
I have no errors and nothing to go on.
I have renewed all provisioning profiles, restored the device, selected "use for development", nothing has helped.
Since there is no feed back from Xcode I do not know what to do.
p.s. The app runs in all of the simulators fine as well.
When I initially select the iPod I see the following series of messages in the console for the devices as shown from the Organizer:
Thu Oct 20 09:42:49 unknown lockdownd[16] : 2ffea000 handle_connection: Could not receive USB message #6 from Xcode. Killing connection Thu Oct 20 09:42:49 unknown com.apple.mobile.lockdown[16] : Could not receive size of message Thu Oct 20 09:42:50 unknown /Developer/Library/Daemons/DTFetchSymbols[308] : Found paths: Thu Oct 20 09:42:50 unknown /Developer/Library/Daemons/DTFetchSymbols[308] : /usr/lib/dyld Thu Oct 20 09:42:50 unknown /Developer/Library/Daemons/DTFetchSymbols[308] : /System/Library/Caches/com.apple.dyld/dyld_shared_cache_armv6 Thu Oct 20 09:42:50 unknown com.apple.mobile.lockdown[16] : Could not receive size of message Thu Oct 20 09:42:50 unknown lockdownd[16] : 2ffea000 handle_connection: Could not receive USB message #6 from Xcode. Killing connection Thu Oct 20 09:42:56 unknown lockdownd[16] : 2ffea000 handle_connection: Could not receive USB message #6 from MDCrashReportTool. Killing connection
I had the same problem, but I solved it. The cause unsuported architecture (by default Xcode4.2 uses standard arch= armv7 ) -> add armv6
In addition to adding the armv6 architecture, I had to remove the armv7 entry from my apps info.plist "Required Device Capabilities" entry.
来源：https://stackoverflow.com/questions/7838154/xcode-4-2-cant-debug-ios-4-2-1-8c148