


If neither of the above is the case, then your hard drive has bad sectors, again this can have many causes, but hard drives are more prone to bad sectors the older they are. Try chkdsk in cmd to check your filesystem. Most commonly filesystem is corrupted, fragmentation can cause filesystem corruption, so can sudden power failures. If not during downloading, then the fault is on your machine. If network conditions changed for any reason, for example, disconnected for a while, the file will very likely be corrupted.

A lot can happen during the file transfer process. As how did it happen, my first guess is during the downloading process. Of course it matters, they should have same size, content and hash, the fact they don't, and the file M is fine, clearly states that W is corrupted, all is clear.
