Go into power settings and turn off fastboot then power off phone. Boot into recovery by holding volume-down key and then holding power key until boot menu appears. Volume down to RECOVERY and tap power to select. HTC logo appears followed by a blank, dim screen. Now, tap all three together: power, volume up and down, and release immediately. You may have to repeat this until you get the timing just right. Black screen appears and sd card error message scrolls. After about ten seconds, recovery menu appears (all in very small font). Volume-down to "wipe cache partition" at bottom of list and tap power key. Cache gets cleared. Then select "reboot system now" by tapping power key again. Done!
Sent from my AT&T HTC One