'Update' volume appeared after High Sierra install This is a spin off from another question. I recently got a 2018 15 MBP with Big Sur on it. I erased the disk and installed High Sierra as I need to run some old software. The install seemed to go fine but a curious extra volume appeared on the Desktop after the install called 'Update' which seems to contain NVRAM and firmware related files.
The volume is mounted normally and not hidden.
I can find virtually no reference to this happening online anywhere.
My guess is it is created as some sort of temporary volume to do with the installation and it should not be there afterwards?
I followed Apple's instructions about erasing an APFS disk prior to installing High Sierra, but these instructions seemed incomplete in hindsight as they suggest just erasing Macintosh HD if other partitions are not available. As I was new to APFS and its partition scheme, it seemed later a better thing to have done was 'Show devices and then erase the device at the top of the hierarchy and I wonder if what I did helped created this mysterious Update volume ?
Anyway what is this volume and can it be safely deleted ?
Note: This answer assumes the reader does not have a Fusion drive.

When selecting the Erase button on the top of the Disk Utility window, you should see a popup window similar to the one shown below.

If instead you see a popup window is similar to the one shown below, then you are not erasing the entire drive. If the drive is not visible on the left side of the Disk Utility window, then press the key combination ⌘2 to show all devices.

When erasing the entire drive, you should select APFS for the format and GUID Partition Map as the scheme, as shown below. (Although, if you chose Mac OS Extended (Journaled), then the macOS installer would automatically convert to APFS.)

From the way I read your question, I do not think you erased the entire drive. You would probably be better off starting over and erasing the entire drive. This should eliminate the Update volume.
Also, consider this: Big Sur uses a newer version of APFS than is known to previous versions of macOS. So if the APFS container was created or updated by Big Sur and all you did is erase a volume before installing High Sierra, then the verify and repair software included with High Sierra may not function properly. So if you need to verify or repair APFS, you should first boot to a Big Sur version of macOS recovery. This is why you would be better off starting over and use the Disk Utility that came with the High Sierra installer to erase the entire drive.