So if you are good today - make block copy of EFS and tar copy using adb:
dd if=/dev/block/mmcblk0p11 of=/sdcard/efs.sbn
tar cvf efs.tar /efs
Also there is a way of saving NVRam into recovery. Do it now if you haven't yet:
adb reboot nvbackup
Then if you still don't feel safe download QPST tools from IMEI thread and backup your NVRam to PC.
My earpiece died and I got a replacement SGS3.
Something (CWM6?) caused IMEI corruption. I didn't know that, so I tried reflashing modems and ROMs. I was trying everything to restore my connectivity. It somehow didn't help that there are seversal guides on how to do IMEI restore.
While I was waiting for another tool download I decided to restore efs from my previous SGS3 on to current SGS3 (I was at my wits end). And I didn't backup current EFS - my huge mistake! But I already almost give up on the phone at that point.
QPST+Peoplearmy tool was able to restore my IMEI. But EFS is now corrupt (why? I cannot even mount it!). And I can only use one modem - the one I used when backing up EFS (UCALG1 modem).
On the good side - after formatting EFS and making it clean, I can run CM10.1 without any issues. My serial number is 000000, but who cares.
Here's how to format EFS:
1. Reboot into recovery (CWM 18.104.22.168 touch is my preferred)
2. You should be able to see phone connected to PC in adb (recovery mode)
3. Issue following commands:
mke2fs -T ext4 /dev/block/mmcblk0p11
mount -w -t ext4 /dev/block/mmcblk0p11 /efs
Lesson learned - try not to mess with EFS at all. Do IMEI restore using QPST and Peoplearmy. I'll keep you posted on EFS restore efforts.