Since there is a lot of confusion about the firmware zips I wanted to give everyone a quick run down about what they are and how they can be used.
Telling Firmware and Update Zips Apart
HTC uses two main types of zip files for their updates. Update zips are the ones most people are familiar with and can be flashed from a custom recovery, while Firmware zips are used for lower level flashing like radios/baseband or bootloader updates. An easy way to tell them apart is by looking at the name, most firmware zips will start with the devices designation such as PJ75IMG. If you open them up you'll also notice firmware zips only contain a handful of large files while update zips contain file folders and their content normally.
Flashing Firmware Zips (If your device is S-ON skip to the next section)
There are three ways you can flash firmware zips:
- The first is to rename the file to 'PJ75IMG.zip' (careful not to add zip twice if extensions are hidden), placing it in the root directory of the sd card and then to restart the phone while holding volume down. This brings up the bootloader which will automatically look for the file and try to flash it. Once it completes be sure to rename the file so the phone doesn't keep trying to reflash it.
- The second way is by command through the recovery. On a S-OFF device, and a handful of HTC Dev unlocked devices, an update zip can pass along a firmware zip to the bootloader and have it flashed once the phone is rebooted. This is how normal updates are installed, however, without S-OFF the bootloader will only accept the request if the firmware zip is from HTC and has not been modified.
- The third way, using fastboot, is my preferred method and usually the quickest. Simply place the the firmware zip in the folder with fastboot and use fastboot as follows
As long as you point it towards a valid file fastboot is not picky about the naming.
fastboot oem rebootRUU
fastboot flash zip PJ75IMG.zip
Updating Firmware and S-ON/S-OFF
All RUUs and OTAs use firmware zips that have been signed with a special key from HTC. This allows the device to know if it's been modified.
When a device is S-OFF it ignores this key and allows you to flash any valid firmware zip, but S-ON devices will only accept them when the bootloader is relocked and if the firmware zip has not modified in any way.
Since firmware updates almost always contain hBoot (bootloader) upgrades to prevent people gaining S-OFF I don't recommend ever updating the firmware on a S-ON device.