If the Security (S) is turned On, the NAND portion of the device is being locked and it can’t be written to. As a factory restriction, when you buy a handset, it has S-On so you can’t have full access to its system.
To find out if a HTC Android phone has S-On or S-Off, boot into Hboot (See http://www.hovatek.com/forum/thread-555.html ) . At the top, you'll see either S-On or S-Off .