I have a Mid-2010 Mac Pro with an Apple RAID Card and a RAID 5 array. On occasion, the system has trouble booting from it's RAID 5 array. The bootup issues have historically been overcome by holding down Option and selecting the boot drive. This computer is listed as compatible for the upgrade to High Sierra. I checked the RAID Utility today prior to updating to High Sierra and found that all four of the drives are in good shape and the array itself is in good shape. I then attempted to upgrade to High Sierra using the download from the Apple App Store inside of Sierra. The installer required an EFI patch to update the system to High Sierra. I ran the EFI patch as per the instructions on my screen. The system shut down. I held down on the power button until it flashed and the system emitted a tone. The installation of the EFI patch completed and the system rebooted. I got a crossed out circle on the screen. I rebooted and held down the Option key to select the Macintosh HD option. The computer finished booting then allowed me to attempt the installation of High Sierra. After clicking to install the system rebooted. I got a crossed out circle on the screen. I held down the Option key to select what was the "MacOS High Sierra Installer" option on the screen. That installer never launched, I again got a crossed out circle on the screen again. I rebooted again, and again, trying to get the High Sierra Installer to work. I unplugged every USB accessory including my Time Machine drive. No matter, if I didn't hold down the Option key it wouldn't boot. If I did hold down the Option key the system only found the "MacOS High Sierra Installer" option to boot from. The option to boot back into Sierra was not there. I tried using my other Mac (a MacBook Pro) to create a USB installer of High Sierra as a backup to boot from but the USB thumb drive I used wasn't detected by the Mac Pro. I then tried to use Rescue Mode to repair the partition of the array. It completed a repair without issue. I rebooted and again got a crossed out circle on the screen. I again booted back to Recovery Mode. The RAID Utility seems to be stripped from Recovery Mode, despite the fact I remember it being here years ago. I used the Disk Utility to erase the partition (I have both a Time Machine backup locally and I use an online backup service, so I wasn't afraid of losing the system's data.) Then I tried to use Recovery Mode to reinstall MacOS. The system told me that the latest version to install was Sierra. The installer started, asked me for my iTunes credentials, then said the software I wanted was not available the installer crashed. I used Disk Utility to again erase the partition. I started to restore from my Time Machine backups but it's saying it'll take 18 hours to restore my files. I think this is extreme as I only have about 1TB of data. I stopped the restore and used Disk Utility to again erase the partition. Maybe it's a bad assumption but I assume this is because the thumb drive is USB 3.0 so it didn't get detected by my older Mac Pro? I then created a new install disk from a USB 2.0 external USB HDD and it was detected. I am trying now to install High Sierra from it and it was at least detected. I'll keep the forum updated in case some of this is helpful to other holdouts like me who haven't upgraded to the Mac Mini Pro (aka the Trashcan). --- Post Merged, Sep 26, 2017 --- The USB Installer of High Sierra booted. Unfortunately, the partition on the RAID array was not detected by the High Sierra installer. I do see there is a RAID Utility in the Utilities menu within the USB installer. I bet this tool is there in the Sierra Recovery Mode but I must have missed it earlier. I used the RAID Utility to delete the array. I used the RAID Utility to create a new RAID 5 array. The Disk Utility on High Sierra installer is also not detecting the RAID array. Going to give Apple a ring on this one... I'll report back what I figure out.