Yes it can detect errors, but it can't continue to function correctly (read: return the correct data) because it doesn't know which copy of the differing data is damaged because it doesn't have checksums.
Moreover, if it doesn't always read both copies of the data (which it may well not, for performance reasons), then you have the possibility of silently propagating damaged data to all mirrors in the case that damaged data is returned to an application and the application then rewrites said data.
Compare that to a filesystem with checksums, which, in addition to being able to detect such a problem, could also continue to function completely correctly in the face of it.
Yep. "What happens if you read all the disks successfully but the redundancy doesn't agree?" is a great question.
Mirrors and RAID5: there's obviously no way that `md` software RAID can help, since it doesn't know which is correct. What about RAID6 though? Double parity means `md` would have enough information to determine which disk has provided incorrect data. Surely it does this, right?
Wrong. In the event of any parity mismatch, `md` assumes the data disks are correct and rewrites the parity to match. See "Scrubbing and Mismatches" section in `man 4 md`:
If you scrub a RAID 6 array with a disk that returns bad data, `md` helpfully overwrites your two disks of redundancy in order to agree with the one disk that's wrong. Array consistent, job done, data... eaten.
Moreover, if it doesn't always read both copies of the data (which it may well not, for performance reasons), then you have the possibility of silently propagating damaged data to all mirrors in the case that damaged data is returned to an application and the application then rewrites said data.
Compare that to a filesystem with checksums, which, in addition to being able to detect such a problem, could also continue to function completely correctly in the face of it.