This tutorial is provided for you who need to downgrade your Xperia X10 to 2.1 Eclair. Some X10 users have some problems when they upgraded their X10 to 2.3. The major issue is SD card which not detected after they upgraded their phone to 2.3. One of the solution is to downgrade their phone to 2.1.
Here we go.
Before you do this, make sure you backup your phone data to your PC. You can follow this instructions How to Backup Your Android Data via PC Using 91 PC Suite.
We are not responsible about everything you done to your phone. This tutorial was tested successfully on my X10.
Here we go.
Download Required Files
- Download FlashTool here
- Download the Generic Global 2.1 Firmware here
Instruction How to Downgrade X10 from 2.3 Gingerbread to 2.1 Eclair
1. Install Flashtool
1. Install Flashtool
2. Copy the *.ftf files that you`ve downloaded earlier to firmwares folder from Flashtool.
3. Click "Flash" button from flashtool and select the *.ftf file firmware and click "OK"
4. Relax and wait a while. When the proccess is completed you will get a popup message (don`t click on that!!!)
5.
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a. Unplug your USB cable from PC
b. Power off your phone
c. Press back button and keep your finger on button
d. Plug USB cable to PC
e. Check if the green LED appear
f. On popup message press "OK" button
g. When "Found at USB/........" appear on flash tool you may release"back button", sit down, relax and wait.
6. IF the "Start Flashing" message appear everything`s ok if not...repeat. Sitdown and relax (again 
7. When last message appear unplug/plug USB cable
8. Done