Please see the tutorial by me below. Hope it can help. Facing problem can PM me. I don't mind help as there's a saying "Sharing is caring"
Step on how to flash your Lenovo A850 to S121 ROM and to have external storage option so you can move your apps to SD. Please follow the steps accordingly. I'm not responsible if there's any damage to your phone during the flashing process.
Please download the files as below:
1) A850 S121 ROM that inudes with SP flashtool.exe:
2) USB driver:
Here we go:
1) Power off your phone and dismantle your rear case to take out the battery.
2) Before you plug in your phone via USB please go to "Device Manager" by right click My Computer and click "Device Manager" tab.
3) Stay at the "Device Manager" page and this must be done real quick. Connect the USB to your phone and PC and you'll see a new device pop out by the name of "MTKxxxxx/Mediatekxxxx". Right click and click "Update driver".
4) Choose the driving according to your Windows version. Ex: Windows Vista. Click the file and click next to install. After you've successfully installed the USB driver, unplug the USB cable from the phone.
1) Open the "SP Flashtool.exe" file.
2) Under the scatter loader tab click it and double click the "MTKxxx file"
3) Above that tick the DA LL
4) Press firmware upgrade and wait 5-10 seconds then plug in the phone.
5) The process will takes around 5minutes plus. Wait till a "green circle" appear that means you've successfully flash your device. Be patient and wait around 1 more minute.
6) Disconnect the USB cable, insert back the battery and the case.
7) Turn ON the unit and for first time load will be slightly longer than expected. Wait till a page pops out and you are ready to set your phone according to your Country and Language.
8) Before you download anything, go to storage setting and set the default write disk to "External Storage"
9) Your phone will restart and now it's the time for your to download whatever you want.
Enjoy and hope my tutorial helps.