When time machine backup is running, iTunes playback is sometimes choppy. I have a brand new 27in iMac (i5) and not much else running. I'm new to Mac and not sure what I should expect in terms of performance impact when time machine is running. No other problems with the iMac. I've had it over a month and all good. I'm also pretty sure I've played music while time machine is running before without any issues. Also, time machine backup takes a lot longer than I expect (seems like 10-15 minutes but I didn't clock it). If it's running once per hour, the diff each hour should be small. And I'm not doing much besides playing music, doing some email and surfing the web. Is there a way to see what Time Machine thinks has changed and has decided to back up this hour?