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I created a bootable USB for High Sierra, and createinstallmedia in Terminal confirmed done, but when I Restart a Sierra iMac with Option key held down, it just boots straight to Sierra. USB drive has GUID partition & is format: macOS Extended (journaled).
iMac, macOS 10.12
I would try reformatting your USB, open Disk Utility, highlight the Disk not the indented Volume.
Click on Erase, give the Disk a name,
Format: Mac OS X Extended (Journaled)
Scheme: GUID Partition Map.
Click Erase.
When done try making the disk bootable using the instructions you used before.
If the installation fails a second time try another USB stick.
I would try reformatting your USB, open Disk Utility, highlight the Disk not the indented Volume.
Click on Erase, give the Disk a name,
Format: Mac OS X Extended (Journaled)
Scheme: GUID Partition Map.
Click Erase.
When done try making the disk bootable using the instructions you used before.
If the installation fails a second time try another USB stick.
I may have a bad batch of USB Drives (I'm investigating), but I discovered that this old iMac (late 2009) does not offer Boot options if I press the <Option> Key with a Restart. However, if I shutdown the Mac & do a full Power-Up re-boot from cold, then the <Option> Key does give the correct result.
Thanks for your answer.
I have had this issue on a Mac Mini. What I had to do was completely Shutdown; then power on and press the Option key. This showed me the possible boot drives. I don't know why but simply Restarting doesn't work anymore.
Bootable USB for High Sierra does not work
