i got the same problem, and the answer is that the "official" clockworkmod recovery don't work on all devices...
don't ask me why (i don't know)
so, i've flashed the Modaco recovery (from paulobrien), and it works like a charm !
So for now, flash this recovery (with fastboot)
LG Nexus 14 16Gb powered !!! 4.2.1 Stock
Samsung Galaxy S3 (I9300 intl) 4.1.1 Stock