Well you may call it bad luck but it is not a big issue. One erase block (= group of flash blocks used as whole unit) is 128KB on Nokia tablets which is quite small part of full 256MB. I've seen like 6 770s, one N800 and two N810s and most of them have bad blocks. The failed dd test surprised me too, though. But it makes sense, for reading and writing one should use specific tools - nanddump and nandwrite from mtd-utils package which skip bad blocks. You can get nanddump from my initfs flasher If you still want to measure the speed or just use skip= parameter of dd to read different part of flash.