Originally Posted by ayush29k
Kindly clarify this please, i want to backup my stock rom before going custom.
So inorder to do a complete backup, we need to do a nandroid backup and also backup imei separately?? and then restore both of them when i flash back to stock??
They are two separate issues, really, but you are correct that you should back up both.
Nandroid lets you go back to a previous state, as long as you flash back to the correct ROM before you restore it. It will not let you restore data to your new installation, though. If you want to restore your applications in your new ROM, you can download Titanium Backup from the market which will let you batch save your applications now and restore them on your new ROM (It can also save and restore system data, but don't restore system data between ROMs, it causes all sorts of weird issues).
Backing up your EFS folder is something you should do, but hopefully will never have to restore. Your EFS folder contains information about your phone and should never actually get touched in any way when flashing ROMs. However, things go wrong and sometimes this folder ends up getting corrupted for whatever reason, which can lead to a corrupt IMEI code and no service. If this were to happen, you would restore you EFS from backup. I think it's safe to say most of us have never had to actually do this, but better safe than sorry. You can do a search for backing it up and find several threads about it.
Phone: Samsung Captivate
ROM: Stock AT&T KK4, rooted.