These types of drives will give you the maximum performance. If you decide to use a external drive, you may want to consider a thunderbolt SSD drive. If the internal drive needs to be replaced, you then exchange the bad drive with the drive in the shell.

If the question mark is persistent, and your Mac does not start up normally: Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off. Startup from your Mac OS X install disc and use Disk Utility to verify/repair the disk. A folder with a flashing question mark means that your startup disk is no longer available or doesn't contain a working Mac operating system. Keep these held down until you hear the startup chime at least 2 or 3 times, then release them and let the machine boot. Often, it is possible to put a internal drive in a USB 3.0 shell which can be externally connected to the iMac. To do so, press the power button to turn on your Macbook and simultaneous hold down Option + Command + P + R. An example video of the replacement procedure can be found at this OWC website. You could continue to use the iMac using the external drive or replace the internal drive. The most likely cause of your problem would be the failure of the internal drive.

(You may need to first run the Disk Utility to create a Mac OS Extended (Journaled) or APFS volume.) Once the iMac is up and running, you can open the System Information application and view the status of the internal SATA/SATA Express hardware. Boot to internet recovery and install macOS on the external drive. Use Disk Utility to repair your startup disk. On notebook computers that have Touch ID, press and hold Touch ID. One way to boot the computer to macOS would be to connect a USB 3.0 external drive (HDD/SSD) to the iMac. If the question mark is persistent and your Mac isn't starting up normally: Press and hold the power button on your Mac for up to 10 seconds until your Mac turns off.

Question mark mac startup