It appears that my upgrade to Big Sur 11.1 has trashed my iMac Pro. I cannot boot.

Hi curtthomas,

Thanks for reaching out to the Apple Support Communities. We understand you're having issues with reinstalling the macOS software on your iMac, and we'd be happy to help!

To get started, let's try booting into macOS recovery and reinstalling the macOS using the steps listed in the following resource under, "Other macOS installation options": How to reinstall macOS

Specifically, we'd recommend using Option-Command-R at startup as this should allow you to reinstall the latest macOS that is compatible with your Mac. 

Let us know if that helps, and we'll continue from there.

Have a great day!

curtthomas,

Thanks for reaching back out.

We'd just like to clarify again, are you holding down the Option key (Alt key if not using an Apple keyboard) + Command key + R key when attempting to boot into macOS recovery, or are you just holding down the Command key + R key?

If using Option + Command + R, and still only getting the option to reinstall OS X Yosemite, we'd recommend reaching out to Apple Support directly at this link here: Get Support - Apple

Take care!

Try re-starting in Safe Mode ( SHIFT ) immediately at startup. It will do a Repair Disk, clear cache files and only load Apple Software required to load computer. It will load very slowly - Normal. This may allow the Big Sur to complete the installation.