I've just downloaded the 18-r1 iso to my Mac (Sequoia), checked the hash and used balenaetcher to create a USB drive. At the end of the verification, MacOS tries to mount the new USB drive but fails - says the drive was not readable.
The same error occurs if I try to mount the iso directly on MacOS.
I've tried different mirrors and confirmed that other iso files mount correctly.
Any ideas?
I think thats just because MacOS itself cant read the ISO file.
If BalenaEtcher says the verification didn't fail, your USB drive should work correctly. The same thing happens if you try to open a ChromeOS ISO on Windows. Windows cant open it, but it works completely fine.
Edit: i just read your note on mirrors, thats weird.
Welcome to the Forum!
Did You tried it with a Reboot after creating the USB Stick?
Eventually, I ignored the errors and put it in the target Mac. It booted OK in that.
It's odd, never before have I had the situation where an ISO failed to mount but was recognised by the bootstrap.