My new 21" iMac freeze on wake when the boot drive is external USB3. I did my own extensive troubleshooting but bottom line with a reformatted drive and only the iMac's shipping version of 10.8.2 installed, BT off, WiFi off, and only the USB3 drive and USB2 keyboard connected the issue still exists. I haven't seen anyone post a complain yet, so I'm thinking its isolated to my h/w. I called AppleCare and they ran me though the basics, which I already did -- reset SMC, boot in safe mode, etc. None of that helped so they told me Apple didn't support booting from external drives and politely ended the support session. So what I want to find out is if anyone here has had any success with booting via USB3, and then also being able to sleep/wake normally. Most likely outcome is: 1) A bug in 10.8.2, or 2) My USB3 enclosures use an older controller chip the iMac doesn't like, or 3) My iMac has a h/w issue and I should return it while I can I just need more data points to figure out which one is most probable.