First post from me, hopefully it may help someone.
I just bought a GT3B with the intention of sticking PSX firmware on it, and had read the more recent "Device is protected" posts in this thread during my research.
Figured I would risk buying one despite the reports of the "Device is protected" error ... I needed another transmitter anyway.
Received the transmitter yesterday and today my ST-Link programmer dropped on the mat.
I got everything set up and got the dreaded "Device is protected" message, seemingly no matter what I did.
Started beginning to wonder if the latest STVP software and Win 10 was to blame.
Went over and over through the methods highlighted by Bjorn above and found the same fix on a Youtube comment, "Make sure that the Read Out Protection (ROP) byte is set to off. Open STVP, go to the OPTION BYTE tab, set ROP to off and write out (Ctrl + P). Then, go to the PROGRAM MEMORY tab again and you should be able to write your hex file."
Then I quite by chance discovered that in STVP 3.4.1 Ctrl-P does not seem to function on the 'Option Byte' tab. It only wrote (and updated the ROP) when I hit the Program current tab icon.
So maybe like me some people are happily hitting ctrl-P like the few available fixes indicate, but never think to try clicking the icon/button instead??
Anyway, hopefully this might help someone.
Off to read up all I can on configuring my GT3B now. 4WS is first on the agenda.