Post-Mavericks, it takes 25 minutes to boot up my iMac

Since upgrading my 24-inch early-2009 iMac from Snow Leopard to Mavericks, it now takes 25 minutes to boot up! I get the grey screen; when it finally opens, though, it’s fine. I’ve used Disk Utility to verify and repair the permissions and the disk, but to no avail. I’m 72 and have been using Macs since the first LC models, but I’m definitely not much of a ‘techie’. John Duffield

During the grey screen, OS X is enumerating (checking each connector and port to see which devices are attached). External devices can slow this process down if they take a long time to answer, but OS X ought to give up waiting for them long before 25 minutes. Yours taking this much time might suggest that the faulty device is internal, but if it were, your iMac presumably wouldn’t work fine when it finally did boot.

Unplug any external USB, FireWire or Ethernet cables, except the keyboard and mouse; now try booting. If (miraculously) your iMac suddenly boots normally then one of those unplugged peripherals is faulty and probably needs replacing. Next try booting in Verbose Mode (hold ç+v down during startup). Look for the last thing displayed on the screen before it locks up, and the first thing displayed after it resumes booting. One is the likely culprit. You can try booting in Safe Mode (hold down ß during startup) – if it works then an updated driver for the faulty device may be all you need. Or it could be a hardware fault and you need to replace that device.

Unchecking ‘When sending to a group…’ shows the group name in your emails instead. If your Mac stalls at the grey screen, you can try Verbose Mode for some clues.