CAN I CHECK WHICH DRIVE MY MAC IS STARTING UP FROM?

Your startup drive is chosen in System Preferences > Startup Disk. In most cases you’ll only have one bootable drive. If you hold option while starting up, Startup Manager offers an explicit choice of drives, plus ‘EFI Boot,’ which lets you start up temporarily from your Time Machine backup.

If your Mac won‘t start or persistently acts up, safe mode helps narrow down the cause. It loads only essential bit extensions (additions to macOS), disables extra fonts in case any are corrupt, deletes temporary files, and prevents any items auto-loading.

After starting up in safe mode, you can carry out a number of checks to identify the issue: see ‘How to use safe mode to start up and find problems’, below. Read more about safe mode at bit.ly/safemodemac. If a process of

elimination suggests a kernel extension could be the culprit, check for third-party kexts: run Terminal (via command-Space bar) and type: kextstat | grep -v com.apple Wading through the code, most kexts show the name of their app (kexts called ‘tun’ or ‘tap’ will be for a VPN product). Google any that remain mysterious. Use the uninstaller provided with that app to remove it, along with its kexts.

If your Wi-Fi stops working, click the Wi-Fi symbol in the menu bar at the top right of the screen (if it’s not there, go to Network in System Preferences, select Wi-Fi on the left and tick ‘Show Wi-Fi status in menu bar’). Check that the correct network is selected and showing plenty of bars. Choose Turn Wi-Fi Off, wait a few seconds, then go back and Turn Wi-Fi On. It should connect automatically, but again, pick your network if necessary.

If it’s a wired connection that’s playing up, such as an Ethernet cable to your broadband router, go to System Preferences > Network and select the

connection on the left. Switch ‘Configure IPv4’ from ‘Using DHCP’ to ‘Off,’ click Apply, then switch back on; or unplug and reconnect the cable.

Don’t forget to check your router, which could be the cause of no connection or a connection but no internet. Its status lights should indicate any problem, which can nearly always be resolved by turning it off, waiting 30 seconds and turning it back on. If not, turn to the router’s companion app or remind yourself how to access its web configuration interface from your Mac.

The application firewall in macOS is turned on by default and should stay that way. All it does is prevent unauthorised apps from accepting incoming network connections, a good way to defeat malware that shouldn’t cause any trouble.

If an app can’t access a connection that it needs to, go to System Preferences > Security & Privacy > Firewall, unlock the settings and click Firewall Options to check that it has permission to accept connections. Find out more about the application firewall at bit.ly/apple-appfw.

In Settings > Apple ID, scroll down to Sign Out, then sign in with the new account. If you try to switch again, you may have to wait 90 days to do so: see bit.ly/ apple-assocdevit. Separately, in Settings > iTunes & App Store, you can freely switch the Apple ID used for purchases.