RAID

You actually brought a good question. What are guarantees for xcopy and similar in case of crashes or power failures? Is it copied all-or-nothing? Can it be a partial copy? It it's partial copy, is is from offset 0 to N or is it from offset N to M (e.g. only last 4 sectors out of 16)? What if one sector out of 16 is not readable? What if target sector is not writable?