How to Nandroid backup before Flashing the Stock ROM? Samsung Galaxy Gio

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Hello everyone,

After days of research on internet I've decided to post here my question. After that I've decided flashing my Stock ROM 2.3.6 I've installed CWM version 5.0.2.6 on my phone. But when I switch on my phone in recovery mode I have an error like;
"E:Can't mount /cache/ recovery /command
E:Can't mount /cache/recovery/log
E:Can't open /cache/recovery/log
E:Can't mount/ cache/recovery/last_log
E:Can't open /cache/recovery/last_log

And I found out that it's because I have a stock ROM on my phone. That's ok till here..
My questions are;
1- If I want to make a Nandroid back up to turn back in case of failrue to not brick my phone, what can I do? because CWM version 5.0.2.6 doesn't work.
2- If I flash my stock ROM accepting the risk of bricking the phone, CWM would work considering the " Can't mount/cache/recovery/command" error ?

 

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Ps: If I install ClockworkMod Recovery Based 5.0.2.8, would it be possible to make a nandroid backup on stock rom and later flash it to CM 10.01?
Thanks a lot!