I'm trying to install Cyanogenmod 11 onto an unlocked HTC One, but when I went to boot into recovery mode, all I got was a red error triangle. The docs don't mention it anywhere, but other reports of this error suggest that I first need to flash a custom recovery image.
So from there I discovered Clockworkmod and installed their ROM Manager app. It detected my phone version, but when I select "Flash ClockworkMod Recovery", it successfully downloads something, but then reports "an error occurred while flashing your recovery".
How do I fix this?