There is some difference with the buttons on the side of the new model. The volume controls are now made up of two individual 'volume up' and 'volume down' buttons and the entire edge is gold-colored (HTC calls this dual-finish metal unibody design). The power button has been moved from the top of the phone to the side under the volume buttons.
On the back, the Duo camera is gone, as expected.HTC says that space limitations prevented it from offering the second sensor on the new model. The 4MP Ultrapixel camera employed on the back of the HTC One (M7) and HTC One (M8) is now pressed into duty as the front-facing camera on the HTC One M9. A 20MP rear snapper is now found on the back of the new flagship device.
Remember, HTC's Jeff Gordon has said that there are things about tomorrow's media event that we have no idea about. The manufacturer is expected to reveal the HTC One M9 Plus and a smartwatch tomorrow. So keep it right here for up-to-date coverage on what is happening at MWC, as PhoneArena will be live in Barcelona!
source: AndroidWorld (translated)