DON'T Downgrade from 4.2 to 4.1. Bad things.

4.2, in order to support the multi-user environment, alters the folder structure for a lot of things. It moves stuff from "/sdcard" to "/sdcard/0". Since this is the "internal" memory, it includes all your non-ROM files (pictures, documents, downloads, music, etc.). They get moved.

IF you decide to step back down to 4.1 (which I did) the OS can't find this stuff. I didn't even realize what had happened until I used Diskusage (one of my favorite apps, BTW) which showed me that I had a bizarre duplicate folder structure within a new folder called "0". A quick google led me to this page at TWRP.

So I tried to use FX File Explorer to move the files, but I ran into a no free space issue for some reason.

MTP is also completely s*** and useless, and trying to copy large files just gives Windows a heart attack and doesn't accomplish anything.

In the end, I had to delete the downloaded "extra data" for some larger games to free a couple gigs of free space, which FINALLY let me start copying the folders back out of /sdcard/0 to /sdcard. Tedious and time consuming and a PITA.

Careful fiddling with 4.2 if you're not sure about your ROM's stability. If there's a chance you might want to downgrade, I HIGHLY recommend backing up your internal storage IN ADDITION to the usual NANDroid backup to be sure you don't lose anything. Normally I'm pretty good about this, but of course this was the one time I didn't.

What phone are you using? Custom ROM?

I have the Bionic and have yet to upgrade from GB to ICS, even though it keeps prompting me to. If you've followed any motorola news then you know that they pushed the update back many times since the release, and at this point i'm worried that if there are bugs, will they be attended to? Or handled like the OS update. Also, i've seen where if a user tries to go back to GB from ICS, it will perma-brick the phone. I was really excited about 4.0 on the Bionic, but now i'm just hesitant, because my phone still works, while others that have updated have been greeting with frustration and bugs.

Thanks for the heads-up!

This was on my Nexus 7. I also have an HTC One S. Both are running AOKP 4.1.2. I had tried an unofficial 4.2.1 update on my N7 but had issues with my Gapps download being corrupt and wasn't near a PC, and also decided I wasn't in the mood to spend all day getting my settings and apps set back up again after a clean flash, so I restored my 4.1.2 update, and that's when I ran into the issues above.

Things got even more exciting after I posted this thread, though. The tablet was running badly, apps were force-closing left and right, and boots took a long time. I tried to reflash and got stuck in a bootloop. Like a moron, I did a factory reset through TWPR which wiped my recovery. Still not near a computer, I didn't have access to FastBoot/ADB to do an external boot to recovery. So I shut the thing off through the stock bootloader and left it alone. Now that I'm back home, I've booted TWRP through FastBoot, reflashed the 4.1.2 update, and am re-flashing TWRP. This time I'll do a fresh 4.2.1 flash with a good GAPPS and hopefully I'll be set.

OEM updates are usually pretty crummy. They take forever to arrive (IF they arrive) and often still end up with major issues. And all that on top of the bloatware and skinning they add to stock Android.

From what I understand, the Bionic is particularly hard to hack because Motorola's bootloaders are tough to crack and Moto isn't releasing an official unlocker for the phone. That's probably why you're hearing of so many problems. Most phones are cake to mess with once the bootloader's unlocked, and even if you do screw something up, FastBoot and an ADB connection to a computer will save your bacon in most cases.