High Sierra and Fusion Drives
Is it safe to upgrade a late 2015 iMac with a fusion drive from Sierra to High Sierra? I tried that eary November and it took the iMac down that requirked Apple Store to fix.
iMac
Yes, as long as you don't try to reformat the drive to APFS which does not work with Fusion Drives. Please see Basic Steps for Upgrading OS X and How to download macOS High Sierra.