This is a very common problem, unfortunately the RS
RS 800 is not suitable for the iMac 27". The iMac 27" can not work with a UPS that doesn't output a sine wave, the power supply is sensitive to this.
When the UPS is in normal operating mode, it's like a power bar and passes utility power to your Mac. When the UPS goes on battery, it generates the power from the battery and outputs a simulated or a square wave. The Mac power supply reacts by making noise and restarting or doing some other strange things.
To solve your problem, you need to get yourself a Smart-UPS that outputs a sine wave. For the iMac 27" which consumes around 350W, you need to get yourself a minimum of a
APC Smart-UPS 750VA LCD SMT750. This unit outputs a sine wave and will not make your iMac 27" have difficulties while on battery power. The 750 will provide you around 9 minutes of run time to safely shut down your Mac.
If you need a bit more run time or have additional items you'd like to back up, you need a unit like the
APC Smart-UPS 1000VA LCD SMT1000. This unit outputs a sine wave and has all the features of a data center UPS, but also has 670W of power. With this unit and the iMac 27", you'd have a run time of 19 minutes.
Either solution would work, as long as you get yourself a back up that outputs a sine wave. The little APC RS units are great for small PC's, but don't work with the Mac's.
Message was edited by: JoeJoe2009