I just bought a new Mavericks IMAC 21.5 10.9.4 (just updated software on June 30) and was having the shutdown problem. I had the machine for 5 days and it started doing the same thing. I spent about 5 hours with senior tech trying to diagnose the problem. At first it was thought that I may have migrated corrupted files or incompatible apps from from my older IMAC 10.6.8 Time Machine backup. We tried a few things, nothing seemed to change, so we erased everything, and reinstalled the new OS without adding anythinhg from the backup drive and left Time Machine off. Still kept shutting down in the sleep mode, needed to press the power button to start, pressing the keyboard or mouse did nothing. We then set up a partition with just the new OS running, tried that and it still shutdown, got so bad that I had to unplug the power cord. It still did the shutdown thing. I returned the computer to Best Buy, and got my money back. Then went to the Apple Store the same day and bought another new one Apple was more expensive but has a better replacement policy. Got the new one home, ran it to set up my internet and e-mail, ran OK, and later just installed a few new compatible software items, no periphials plugged in but had a 10 port USB 2.0 (powered from wall unit) and a few files from my external drives. Everything seemed to work OK, then it seemed to to the same thing the first day. After reading all the other discussions about this matter I started thinking about the comments about the USB ports. After trying to be logical about what was happening it seems that there may be two things happening? Because other users said they also had to do a SMC reset I realized that it was some sort of power problem since it caused sleep and restart problems mentioned by other MAC users. Here what I think may be happening and why: (1) It could be a USB compatibility problem because the external powered hub I was using was USB 2.0 and it was plugged into USB 3.0 ports on the new MAC? (2) The powered USB 2.0 10 port hub (about 5 years old) was defective possibly causing SMC problems? (3) Perhaps the power from the hub caused a nasty reaction back into the SMC controller circuitry? (4) Maybe the USB 2 hubs and plugged in devices were incompatible with the USB 3.0? Here's what I tried: removed the 10 port hub and tried with just my Maverick compatible printers installed into the IMAC ports directly. So far in 24 hours the problem hasn't returned. Although when I did a MAC software update today after it finished the installation it went into the sleep mode, then rebooted OK when I pressed any key. So I think this shutdown and sleep problem may be USB or some sort of SMC problems it may causing? I am going to try just using my USB ports on the new IMAC, and only plug in the device that I know are compatible with the new OS and see if I can isolate the problem. I just wanted to mention that I have an older 10.6.8 IMAC now sitting on the floor that I seemed to have a similiar problem causing repeated kernal panics combined with the SMC and power reboot problems. I took it to the Apple store and they said I had bad memory, and when they ran their test after reinstalling a clean OS for 24 hours they were a little stumped and said either the memory was defective (they suggested removing the old and trying new memory chips) or that the mother board was bad. I had the memory checked, it worked OK at the technicians home, unplugged it brought it home about a hour later. When I sat it on the table with absolutely nothing (no power cord!) plugged it, it started chiming about 10 times. I was unable to restart the machine, after trying tying ALL the procedures. I haven't given up yet on that one. I am starting to think it may be the external hub or USB3 compatibility that was causing the power problem on the old IMAC and the two new ones because I was using the hub. I ordered 3 different brand new quality? USB 3 unpowered 4 port hubs and going to trying them and see if anything changes, and unplugging periphial devices one at a time and see if I can isolate the problem. I may be missing something here but because this problem seems to be happening with other MAC users and seems to be a power related problem the culprit may not be a software problem, nor an internal MAC problem, but somehow the logic board or SMC controller is being corrupted by a USB hub or USB device as another user suggested earlier. Maybe some other users can investigate this and see it it may help isolate the problem. I will report back to this discussion group in a few days after trying the new USB hubs etc and let folks know about my results.