I am looking for some advice on why my iMac has become so unstable as to be repeatedly crashing following a clean install of High Sierra. First, some background on the setup. I've been running this iMac off of an external thunderbolt SSD for a few years now and that's been working just fine prior to the OS upgrade. The reason for use of the external SSD is to take the place of a broken fusion drive inside the iMac. The HDD portion of the fusion drive is completely broken...as in making clicking sounds when the unit first powers up. For some reason the unit also takes a lot longer that it used to before getting startup chime/bong and then longer still before I get to the OS. I'm wondering if the broken hardware is causing the issue or it it's something else. Is there any way to completely disable the internal drive so the Mac does not even try to access it first? Thanks in advance for any suggestions.