BillyPhil66,
Replace the drive.
The HD521-2W error is related to the the SMART logs in the hard drive. SMART logs are maintained by the drive itself and keep track of read / write errors, temp and other parameters that may indicate the impending failure of the drive. PC-Doctor reads these logs and reports an error if the drives thresholds have been exceeded (this is all up to the drive itself (PC-Doctor does not interpret these it just reports if a drive thinks it's going bad).
Depending on what SMART log value is tripping it is conceivable that a failing drive could pass and then fail and then pass again. Some counters are cumulative to other counters. For example say it is the temperature value. The drive could be over temperature, tripping that value for that power on cycle, this would then increment a "over_temp" cumulative counter. The next time the system was turned on it would not be over temp, show no error but, be one tick closer to the cumulative temp counter giving a permanent SMART error.
Every drive manufacturer does this differently. It is totally up to them on what values to use and how to interpret it, the standardized part is exposing the data and the reporting of a permanent SMART failure.
The exit engine HD-xxx is is just reporting the test engine is done and exiting.
-Fred