sorodrigo: This worked for me:
1. Backup everything on your SD card.
2. Go into the WinMo bootloader (Vol Down + Power) when your device is starting up.
3. Install any WinMo 6.5 ROM + Radio 2.15.
4. Go back to the bootloader and install Magldr 1.13.
5. Once in Magldr, go to USB Flasher and run DWI.exe from HD2_Updatable_XBmod_Yuki_v1.
6. Once completed, Windows Phone 7004 will boot. Set the correct time and connect your phone to Zune.
7. Zune will setup your phone and look for updates. If you update, your phone will be updated to 7008.
8. After reboot, make sure the time and date on your phone is correct and check for updates again.
9. Zune will show an available update and update to 7392.
10. After the update completes, download the HD2-7720 and WP7 Update Cab Sender packages and extract them to separate directories.
11. Go into the HD2-7720 directory and then go into the update tool directory (x86 or x64) depending on your system and install Setup-UpdateWP.
12. Download DeviceProvisioning.cab and copy it to the WP7 Update Cab Sender directory which contains the file WP7 Update Cab Sender.bat.
13. Run WP7 Update Cab Sender.bat (not as Administrator and make sure Zune is not running) and press S. Wait for the process to complete.
14. Once complete, open Zune and check for updates. It will say update found. Go ahead and complete it and you will be updated to 7403.
15. After updating to 7403, reboot and make sure the date and time on your phone are correct. Then, check for updates in Zune. It should show you an update available to 7712. Update and wait for it to complete.
16. The 7712 update will take about an hour - 1.5 hours to complete, so be patient. Once that is done and phone reboots, you will get a storage card error. Turn off phone at that point and boot again. On the Magldr WPH Boot .. Go Go Go screen, press Vol Up + Vol Down and then press Vol Down twice to reset phone to factory default.
17. When you boot again you will not receive a storage error. Without connecting the phone to Zune, download 1OEM_8x50_NK_SD_REMOVE.cab and 2OEM_8x50_NK_SD_710_COTULLA_FIX.cab to the WP7 Update Cab Sender directory and run the bat file. Press S to start the process.
18. Once the process completes, it will show you that your phone has been updated successfully. Then go to Settings > About and Reset Phone.
19. You now have 7712 with working SD card. Don't proceed to 7720 yet till the certificate issue is fixed.
19. Power off phone and remove SD card
20. Restart and On the Magldr WPH Boot .. Go Go Go screen, press Vol Up + Vol Down and then press Vol Down twice to reset phone to factory default.
21. Install the HD2-7720fix.rar files one by one in this order.
-dpi-262.cab
-lang-***.cab (0409 and 0809)
-pks.cab (note: I had to reset between install files to clear enough memory)
22. After final 7720 package is installed, reinstall SD card and reboot. On the Magldr WPH Boot .. Go Go Go screen, press Vol Up + Vol Down and then press Vol Down twice to reset phone to factory default.