how come my android devices are not visible on my Mac?

I'm new to android development so apologies if this is a stupid question. I'm having trouble seeing my android devices in Eclipse. I've switched on USB debugging, configured them to connect as a camera not mass storage, but when I run ./adb devices, there's no device showing, even though I have a Samsung Galaxy and a LG Nexus connected via USB. On both phones I get a notification saying "USB debugging connected" and "connected as a camera" so the phone obviously thinks it's connected. The problem appears to be intermittent. Sometimes the Galaxy shows up: Macbook:~/workspace/android-sdks/platform-tools ./adb devices List of devices attached 3334AB246B5300EC device Then I unplug and reconnect it, and it disappears. I've tried 3 cables and 2 devices now. Should I just give up and use an emulator instead?