A mi no me funciona, hago todo los pasos al pie de la letra y me sale esto en el Flashtools
04/059/2014 12:59:50 - INFO - <- This level is successfully initialized
04/059/2014 12:59:52 - INFO - Flashtool Version 0.9.9.0 built on 2012-11-13 22:53:30
04/059/2014 12:59:52 - INFO - You can drag and drop ftf files here to start flashing them
04/000/2014 13:00:02 - INFO - Device disconnected
04/000/2014 13:00:29 - INFO - Selected ST21a_11.0.A.0.16_Movistar.ftf
04/000/2014 13:00:29 - INFO - Preparing files for flashing
04/001/2014 13:01:27 - INFO - Please connect your device into flashmode.
04/001/2014 13:01:52 - INFO - Device connected in flash mode
04/001/2014 13:01:52 - INFO - Opening device for R/W
04/001/2014 13:01:53 - INFO - Reading device information
04/001/2014 13:01:53 - INFO - Phone ready for flashmode operations.
04/001/2014 13:01:53 - INFO - Start Flashing
04/001/2014 13:01:53 - INFO - Processing loader
04/001/2014 13:01:53 - INFO - Checking header
04/001/2014 13:01:53 - INFO - Flashing data
04/001/2014 13:01:58 - INFO - Loader : S1_Root_a8a4 - Version : R5D030 / Bootloader status : ROOTABLE
04/001/2014 13:01:58 - INFO - Disabling final data verification check
04/001/2014 13:01:58 - INFO - Processing partition-image.sin
04/001/2014 13:01:58 - INFO - Checking header
04/001/2014 13:01:58 - INFO - Ending flash session
04/001/2014 13:01:58 - ERROR - ERR_SEVERITY="MAJOR";ERR_CODE="000A";ERR_DYNAMIC=" 0x0506: sin header verification";�
04/001/2014 13:01:58 - ERROR - Error flashing. Aborted
04/001/2014 13:01:59 - INFO - Device connected in flash mode
04/002/2014 13:02:00 - INFO - Device disconnected
04/002/2014 13:02:04 - INFO - Device connected in flash mode
espero me puedan ayudar.!!!