On this thread I'll post how it went using encryption on the Galaxy S2 GT-i9100 with MIUI 3.9.27. - Turning on encryption requires: -- At least 80% battery -- Phone connected to charger -- Lockscreen with PIN or Password. - Encrypting a device which is already loaded with data may fail. At least it failed on my SGS2. I had to go to recovery, factory reset and format both internal and external sdcards. I had also to empty /data/lost+found which Root Explorer which almost filled my data partition (1.9GB out of 2GB). So is mandatory that you encrypt your device just after you do a fresh install of MIUI. - Internal sdcard is encrypted (thus can't be mounted in UMS mode from recovery); external sdcard is not encrypted. - Decryption is not possible for either /data or internal sdcard. - Doing a factory reset from Settings doesn't wipe internal sdcard. In fact, it's not accessible anymore. You have to install an official Samsung rom and wipe the internal sdcard from Settings (even from recovery doesn't work). See bug here. - MTP is still working, UMS only for external sdcard, never for internal.