Flashing one Rom over another

marsui · September 7th, 2012, 08:18 AM

I'm still a bit of a noob with flashing roms and stuff, I'm trying to get a better understanding of what happens when you flash one rom over another.

My EVO4G had an unfortunate water damage accident, I had it rooted and was running MikG sense 2.1+3.0 rom. I received a Samsung Transform from a friend as a temporary phone until my upgrade is up in December.

I had EF09 so I had to use SWupdater to root (as opposed to having stock froyo EB28 which allows a one-button root). I played around with a rom called TransGingerOnCrack, but some things were querky, so I wanted to try Ubuntudroid v6.

So I flashed Ubuntudroid, and noticed that all my apps and contacts were all still there. What happens when you flash one rom over another, and why doesn't it wipe out everything? Can anyone give me a brief overview of where things are stored in system memory?

I know in custom recoveries you have the option to wipe all these different things, but I never understood why I need to do so, or why I don't need to, I just follow exactly what the guide says, and don't really question it lol.

Thanks

The_Chief · October 10th, 2012, 08:34 AM

Sure Marsui... I don't have a Transform, but Android is Android.

There are various partitions in the phone's memory that are completely separate from the SD card. The main partitions are:

Boot: this boots the phone. NEVER mess with this!
Recovery: if you can get to recovery, you can save a bad phone. Stock recoveries are limited to what the carrier permits... custom recoveries like ClockworkMod allow the user to replace the operating system (ROM).
Data: (big one) this is where user and app data are stored on the phone. Settings, contacts, high scores, etc. If this is cleared, you lose all your apps and data. A Data wipe is also known as factory reset.
Cache: this is temporary memory used by user apps to run better. It can be cleared occasionally with no problem.
System: (big one) this is where the ROM, or operating system of the phone, is. When the phone boots up, it's completely stupid until it loads the OS from System.
Dalvik Cache: this is essentially a cache for the system, allowing it to run better. It can be cleared occasionally as well, but when you re-boot it will take longer as the phone has to repopulate it.

Glad you asked? Good!

When you flashed the ROM, it replaced the contents of the System partition. If you didn't wipe Data or the two caches at the same time, then your settings and app data will remain (as will -very likely - a lot of software conflicts and hang-ups). That's why it is VERY important, when flashing a new ROM, to do 2 things:

(1) MAKE A NANDROID BACKUP FIRST! If something goes wrong, you want to be able to go back to the state the phone was in at the moment you made the backup. Starting all over is a real bummer, and there's no need to.

(2) Wipe Data, Cache, System and Dalvik when flashing a new ROM. Especially if it's a different ROM. If it's an incremental update of the same ROM (let's say you're going from 2.3 to 2.4), then you might be safe by wiping just System and the caches.

Most ROM zips now include a subroutine that wipes System before loading the new ROM, but it doesn't wipe the other 3 partitions. That's on YOU.

Hope this helped explain what's going on a little bit... if you have any other questions, feel free to ask