[Galaxy S 5 Q&A, Help & Troubleshooting] droidzer1:
If you want to read the below learning experience you may gain some knowledge but at this point I believe my method may have flaws. While it seemed fine at first the problem listed directly below could not be fixed without using Odin to flash a stock ROM back onto the phone. It had checked the hidden menu settings for MTP and ADB and PDA, CP etc and all were correct (I have another good working S5 to compare these values to). I tried an earlier restore of a TWRP backup and that just blew up with all sorts of FC's on boot. So thank goodness for Odin. After flashing the stock rom it booted fine and I can now write files from the PC to the phone again via USB. In the process right now of re-rooting, installing TWRP and doing a restore of my last backup.

Note : I have found one problem I am currently working on after using this method. I can hook up the phone to the PC and get a link, open folders on the phone and can copy file from the phone to the PC but can NOT copy files from the PC to the phone. More later .....

I've got a Samsung Galaxy S5 from US Cellular. Other than a few people in the Motorola subforum here trying this I haven't seen this exact method described here so apologies if I missed it somewhere - tried many searches. It seemed so easy - maybe it is just considered SOP for some - don't know so I'm going to run the details by here anyway in case it saves anyone some trouble. Do read my warnings below though before you attempt this.

1 - You want TWRP installed (or Philz CM Touch will probably work too - seems a bit more complex though) If you are flashing ROM's having TWRP or some other custom recovery is always a good idea and I'm sure most people have one of them.

2 - Always do a complete backup with TWRP before flashing anything new. You did a backup before going to Lollipop didn't you? :p

3 - So you are tired of the white screen, battery sucking, quirky Lollipop or maybe that Cyanogen Lollipop mod didn't quite work for you and you want the old Kitkat back. So fire up TWRP, go into Advanced wipe and wipe Dalvik Cache, System, Data and Cache. You aren't done wiping yet though. After you swiped to wipe those go back to the previous page and Swipe to Factory Reset. That may not actually make any difference but I just did it for good measure and my method worked great.

4 - Now without out even needing to reboot find your latest backup of Kitkat 4.4.2 that you have on your Internal SD or External SD card (you do keep one there don't you in case your phone glitches on you while you are out and about?)

5 - Do a Restore of everything - Boot, Recovery, System, Data, Cache.

6 - Reboot. You are done.

Firstly I need to add that I had flashed Cyanogen Mod 12.1 (Lollipop 5.1) after following directions here in the main Cyanogen subforum to flash that on my KitKat 4.4.2 Galaxy S5 phone. What may be noteworthy is that after flashing CM 12.1 I ran the App PhoneInfo Plus (made mostly for Samsungs) and the OS version showed as Lollipop 5.1 but some of the build numbers to my recollection showed 4.4.2. So this was not a stock Lollipop OTA - it was the Cyanogen mod of Lollipop made by developers here. I just assumed they started with Lollipop to make that latest 5.1 build but I haven't followed the Cyanogen mod much at all so I don't know if they have borrowed bits and pieces of Kitkat in their builds. If they have done that then this downgrade method may not work the same for an OTA Lollipop.

Another warning - I noticed in the Motorola subforum here that some people bricked their phone trying this type of downgrade with TWRP however I believe that is because they had Verizon phones with locked bootloaders and that is not the case for my phone (US Cellular) or any other phones beside VZ as far as I know. If you have a Verizon with locked bootloader beware! I will also add that on my first boot after restoring Kitkat 4.4.2 that the second the home screen was up I started quickly scanning through my app pages and the phone rebooted itself. I believe it wasn't fully done loading the OS and it just was a hiccup. It has not happened since after trying all sort of things. Everything is working great.

I saw most people recommending to use Odin to flash a stock Kitkat firmware back on the phone in order to downgrade from Lollipop. Then you need to re-install all your apps or maybe do a restore of a recovery but with this method using TWRP you bypass most of that and get it all done in one TWRP session.

Another note about what I read people having a problem with upon downgrading - some were losing their IMEI when using Odin to flash the original firmware. I still have my original IMEI with no problem. That is another reason I think this method may be better than using Odin for this downgrade. I'll add that there are a lot of people around here with a whole lot more knowledge about flashing and custom ROM's than me so don't take my ideas as hard fact without doing your own research for your particular phone and particular situation.