SKahan,
The HD521-2W indicates a SMART failure, this is the drive itself indicating it is going bad.
The HD535-2W indicates the test is encountering an error during the surface scan test.
Neither of these are a good sign. I would back up data and replace the drive as soon as possible. Chkdsk /f may correct errors temporarily however the combination of the SMART flags being tripped and the problems reading the entire disk surface point to impending doom.
-Fred