Sometimes iMacs Pro cannot be turned on properly because of external hard drives. After pressing the power button, users see a white screen with an Apple logo, but a sign in screen does not appear. A computer remains in this state until all external drives are removed, and restart process is initiated. Then it boots up as usual. Or, iMacs may require a lot more time to boot up, i.e., 30 seconds or more. In some cases, problems show up after the system upgrade.
If you have the same troubles, try the following.
With Safe Boot, you can find out if the problem is in external hard drives or a Mac itself. To enter this mode, turn your computer on and immediately press and hold the Shift key until a login screen shows up.
Then restart your Mac like you usually do and see if the issue is still there.
Boot your Mac without external disks plugged in and then connect them one by one to see if one of them is to blame. You can also try the second isolation test if the first one does not show any results. Try attaching hard drives one by one before turning on your computer.
Also, you can reconnect external hard drives to different USB ports.
System Preferences Check
Do not forget to check if a right startup disk is selected. The thing is a Mac can start up from any storage device with a compatible OS. It may be that your computer does not know what disk it has to boot from, so it tries to check each of them, which breaks the booting process. It may also be that external drives are asleep, but the system sees there is a connection and waits for them to wake up. Then a freezing issue most probably reappears if you turn a computer off.
You need to set your internal hard drive as a startup disk.
- Choose the Apple menu and then go to System Preferences to select Startup Disk.
- Enter an administrator password.
- Select your startup disk.
- Restart iMac Pro.
Use this built-in tool to verify your iMac Pro startup disk without starting up from another one via Live Verification. If Disk Utility finds any issues, you will need to start up from your macOS X Install DVD and run Disk Utility on it to repair damaged parts of the startup disk.
Keep in mind that you may not be able to cancel Live Verification once you start it.
Also, if you have not updated your macOS since Yosemite, you can use Disk Utility to repair disk permissions on a startup disk.
- Open Disk Utility through Spotlight or Finder (you can find it in the Applications folder).
- Choose a startup disk from the list of volumes and click the First Aid
- Select Verify Disk Permissions. Click Repair Disk Permissions if needed.