[Thinkpad Tablet General] grimsgrams:
Dummy's Guide to Root and exFAT support

Background: Having no Android experience I struggled far too long with getting this to work. Here's a quick and much more detailed foolproof guide. Not every step has to be followed precisely (i.e. in terms of connecting cables and turning it on and off) but at least this should work.

Who and why: You have a Yoga Tab 3 Pro, and you want Android 5.1 (the latest version available), root access, a speedup and support for a MicroSD card with exFAT. exFAT lets you have files larger than 4GB. There are some inherent risks, but if you avoid picking the obviously wrong menu options it should be perfectly safe. Being a complete dummy, you are a Windows 7, 8 or 10 user like myself.

Sources: for Rooting: inioni and joesnose. For speedier unencrypted data partition: inioni and joesnose. For Exfat: lurker00 and Tzul

What you need: A MicroSD card, a converter or something to make the card readable on your PC (possibly a regular-sized SD card adapter), a USB cable to connect your PC to your tablet (probably regular USB to microUSB).

Step 1: Charge your tablet to 80%+. When it's running, go to About Tablet, then System Update, and update. Android version should be 5.1, security patch level 2016-03-01.

Step 2: On completion of this step you have a folder with 4 relevant files. To do that: create a folder on your PC for storage of a couple of relevant files. E.g. C:\Yoga. 1) Download twrp.img.zip from the bottom of this post and extract the .img file inside to that folder. 2) Download x90-disable-encr.zip from this post and place the zip file in the same folder without extracting it, ignore the file starting with twrp on the same post. 3) Download UPDATE-Android-fs-1.0.4-install.zip from here and place the zip file the same folder without extracting it. 4) Download UPDATE-SuperSU-v2.46.zip from here and place the zip file in the same folder without extracting it.

Step 3: On completion of this step you have a microSD card formatted as FAT32. To do that: if your microSD card is 32GB or less, you rightclick on it in Explorer, select Fat32 and Format. If your microSD card is more than 32GB Windows will not show the FAT32 option, so you have to use a utility like e.g. this one.

Step 4: On completion of this step you have the program FastBoot installed. To get that, go here, select one of the top options under Download, run the file. The install process isn't usual, just type Y and enter. Fastboot doesn't look like a normal Windows program with an icon and stuff, it's installed after it says that it's installed.

Step 5: On completion of this step you have enabled OEM Unlocking on your tablet. To do that: go to Settings on the tablet, About Tablet, press Build Number several times. If you are unsure of which one is Build Number (for example: different language), you should get a notice after hitting it 4 times. Just tap each of the options 7 times until you find the right one. Then go back to Settings, choose the new Developer Options at the bottom, enable "OEM Unlocking".

Step 6: On completion of this step Fastboot is able to communicate with your tablet. Connect the tablet to your PC with the USB cable. Restart the tablet. Open a Command (CMD) window on your PC, e.g. by rightclicking on the windows icon bottom left and choosing Command Prompt, or typing CMD in the search box. Type 'fastboot devices'. If you see a line with an identifier, you're all good for this step. If not, you are probably missing Android USB drivers for your PC. Download those from here and install them, or follow instructions here. Restart your PC. Reopen a command window. Type 'fastboot devices' again. If you get an identifier this time, you're good. If not, stop following this process and ask for help.

Step 7: On completion of this step Fastboot has unlocked your tablet. To do this: First copy off any user data you want to preserve from your tablet, as this wipes all user data. Keep the tablet connected by USB cable. Restart it, and hold the volume key furthest away from the power button. A new screen pops up instead of the usual Lenovo logo. In the CMD window on your PC, type "Fastboot oem unlock". The tablet prompts you to accept and delete everything, do so.

Step 8: On completion of this step you have a safe backup of your tablet in an unmodified state. Turn off your tablet by holding the Power button for several seconds and insert the microSD card. Restart the tablet and hit the Volume button closest to the Power key, to enter the Fastboot environment. In the Command window, navigate to the same folder, e.g. by typing "cd .." then "cd Yoga". Type "dir". You should see the four files from step 2. Type "fastboot boot twrp-2.8.7.0(6)-x90.img". After a brief delay, your tablet should restart with an unfamiliar menu system. Slide the blue slider to the right, select Backup. In the backup menu, choose all the folders, select storage, select your microSD card, and run it. After it finishes, exit out using the home button on the screen bottom, and Power Off. Disconnect the tablet, power it off, remove microSD card, insert in computer, copy the contents to your Yoga folder. After this is done your PC Yoga folder should have both the four files in it, and several backup folders. That's your tablet backup.

Step 9: On completion of this step you have rooted your tablet. Copy the four files from step 2 to your microSD card, so it now contains both the backup and those files. Insert it in your tablet. Restart your tablet, hit the volume button closest to the power button to enter the fastboot environment. When connected with a USB cable, in a CMD prompt in the Yoga folder, type "fastboot boot twrp-2.8.7.0(6)-x90.img" to return to the TWRP environment. This time, select Install. Install the UPDATE-superSU-v2.46.zip file. When finished, press Home button, disconnect USB and restart the tablet.

Step 10: On completion of this step you do some stuff which might be unnecessary, but go ahead and do it still (I did, and it works here, so hey). When you restart the tablet, hit the volume button away from the power button to boot normally. You should get a Lenovo logo, and after waiting a while for something to happen (possibly up to several minutes) enter the first time user setup. Just go through it. Don't do anything else. Restart the tablet, hit the volume button closest to the power button, reconnect USB.

Step 11: On completion of this step you have done the first part of installing the unencryption script and the exFAT file system. Run TWRP again using the method in step 9. Go to Install, install the two zip files "x90-disable-encr.zip" and "UPDATE-Android-fs-1.0.4-install.zip" at the same time or one after the other. Restart the tablet, again going into the Volume-closest-to-power menu. In your Command window, run "fastboot format data". Disconnect USB, restart tablet again, use volume button away from the power key. This time the Lenovo logo should come up again, and it can take up to several minutes until something more happens and you enter new user setup. Complete that again.

Step 12: On completion of this step you have exFAT support. Install the app Busybox. Open the SuperSU app, go to Settings and DISABLE the option "Mount Namespace Separation". Install and open the app Root Browser (any generic file manager which shows Root files). Say Yes to any SuperSU permission prompt and No to download tools/updates. In Root Browser, hit the House button top left. Go to the /system/etc folder and spot the file named "install-recovery-2.sh" (note -2 at the end). Select it then Copy. Hit the green arrow at the top to go back to the System folder. Use the blue New cross to create a new folder called /system/su.d. As in, create a folder named "su.d" with a single dot in the name under System. Inside that one, paste "install-recovery-2.sh"

Step 13: Success! You can now use any exFat card in your tablet, at least as long as it's present when the tablet starts up. To reformat your microSD card, download the SD Card Formatter, in options select FORMAT TYPE: QUICK and SIZE ADJUSTMENT: YES.

OTHER STUFF

The TWRP environment from the "fastboot boot twrp123xyz.img" command is just a temporary one loaded on the tablet using Fastboot, and disappears from memory when restarted. To do it you have to be in the Fastboot mode i.e. hit the Volume button closest to the power button. To normal boot, hit the volume button furthest from the power button.

When backing up, just backup all the folders (someone correct me if wrong).

To restore a tablet backup from inside TWRP, go to Mount and uncheck "Mount system partition read-only", then Restore.

Optionally: With Root alone, using an unencrypted file system is not necessary, and you don't have to install exFAT either. You can simply root and use the Paragon NTFS app which lets you mount an NTFS formatted microSD card, also taking files larger than 4GB. However, this is not as clean - the tablet believes it doesn't have a microSD card inserted, and Paragon just mounts it to an ordinary folder. So e.g. in Windows over a USB connection you are informed your tablet has 20GB free, but you can dump a bottomless amount of files only in this specific folder. Using the exFAT method above instead it works just like a regular microSD card.

Let me know if any of this is egregiously wrong and I'll change it.