For all amateurs like me:
The trick is to RE-LOCK the device from a PC !!!!!!!!!! And only after that you can run the GB flash from SDcard.
Now the question is, what is the exact sequence of next steps since there is not one FULL guide around here.
Since no pro here wants to spend his time writing it, I'll try to invent one and I just ask for approval or/and corrections
After being back on GB, Re-locked, S-ON (still don't know if I need to flash the stock recovery too ???)
1. Unlock device via htcdev
2. Install (from SDcard or adb?) recovery(exactly which custom recovery corresponding to the step 5?)
3. Root GB (from SD or adb? or not needed at all?)
4. Install 4ext from the market?
5. Follow to