this is also happening with me. The device is attached. And so all the internal actions are getting logged and the error message disappears very quickly. You should try the following (it worked for me)
- Restart Eclipse (as stated here Why logcat is not showing anything?)
- if 1 does not work, you may try to increase the logcat message size. From Window > Preferences > Android > LogCat (increase size to more than 5000)
First one worked for me. In this way also you need not to detach the device everytime you deploy the app from eclipse. Regarding, the other question - you can always clear the existing log from [Clear log] button.