# Can unwritten (but presumably otherwise usable/functional) blocks on an SD ever fail to be readable?

Does an SD come from "the factory" with all of its ECC (or spare area) set correctly?

Sometimes when I blindly dd (copy) an entire SD card (that is only partially filled with filesystem info/data -- and otherwise has lots of unused/never-written-by-me raw space) to another one, I will get a (repeatable) read error on a block that is seemingly in a part of the SD that was never written (since it was close to the "end" of the partition).

Since the source SD was otherwise functional, I've always assumed that this read error was indicative of the block having never been written (and thus it didn't have appropriate corresponding ECC). Is this a bad assumption?

[Now that I type this "out loud", I'm thinking that, since there weren't a constant stream of read errors, the error-causing "unwritten block" would need to be somehow special -- perhaps it was a remapped block and thus didn't get an associated ECC?].