Since OS X 10.6 will have all kinds of Wake-on-LAN support built-in, I thought I'd start playing with it, and I discovered something weird.

Setup: APC UPS Back-UPS ES 550, purchased maybe a year ago. Mac Pro and external FW drive plugged into the battery-backed plugs. Other random crap plugged into the "surge-only" plugs.

Apple menu --> Sleep. Computer powers down and immediately powers up. Quoth the system logs: Okay, fine. Sleep again. This time, the computer powers down. The Firewire drive spins down. All is well.

Thirty seconds later, the UPS gives a continuous beep and the computer refuses to wake up. It looks like the UPS cut power to the computer. Hitting the power button on the UPS then powers it down. Hitting the button again starts the UPS up fine and the computer reboots. If I try to sleep the computer again, the process repeats exactly as above.

According to the APC manual, the only listed condition for which it would do a continuous beep is if it wants the battery to be replaced, but this would also include the "Replace Battery LED" flashing, and it's not doing that.

I do have the UPS plugged into the Mac via USB. The Mac reports that the UPS is at 100% charge and is plugged in.

Any ideas what's going on? Any thoughts on further experiments to try? Ideally, I'd like my computer to power down the first time tell it to go to sleep, and I'd like this to not freak out the UPS.

I have an APC 750 ES UPS with a special labeled plug on it that it uses to sense power. If power consumption drop (IE, the computer turns off or goes to sleep), it will cut power to a few of the other plugs. I found that when I had my main monitor on a plug that would lose power, the USB hub dropping off was enough to make the Mac wake back up from sleep. I believe my UPS shipped with this turned on by default, so it might be something to double check on yours. Overall I find it handy since it does kick in when the Mac is put to sleep or turned off completely.

Following on with what Tom mentioned, try the same steps without the UPS plugged into the Mac's USB port.

Removing USB devices from a sleeping Mac will definitely wake it up.