I made an install USB with Fedora Media Writer for Silverblue 40. When I try to boot from it, I get an error saying “secure boot violation.” I’m using a Gen 12 ThinkPad X1 Carbon. Has something changed with Fedora that you now have to disable secure boot to install it?
That’s weird. You might be hitting Media Writer produces USB stick with FAIL media check · Issue #669 · FedoraQt/MediaWriter · GitHub.
Well, it didn’t go well.
I disabled secure boot and installer anyway. The installer seemed to function but when it was done and I rebooted, it took me back to the BIOS looking for a boot manager. The Windows boot manager was gone, and Fedora didn’t seem to install one. Not sure if this is unique to me or a bug in your installer. Left me with a bricked laptop.
Lucky I had a Windows recovery USB. Re-installed Windows. Typing this while it goes through endless updates. 
What OS and what app were used to create the usb device?
Was the usb device ever plugged into a running windows machine after creating it?
It has been found that windows alters the device by writing to it when plugged in and that corrupts it for booting.
I have read somewhere – don’t remember where – that some computers can disable the secure boot certificate that is used to sign the linux boot loaders. If your machine is one of those, you should be able to enable the usage of that certificate.
I used Fedora Media Writer on Windows 11. So yes, it was plugged into a Windows device. I can try creating the USB again using macOS.