Anytime you initiate a hardware test on Mac OS X connected hard drives and in return receives the result as “Mass Storage: Error Detected 2STF/8/3 : ATA - 100 ata - 6 – Master”, then consider something fishy with the Macintosh HD, and a serious attention is required in the case.
To begin with the occurrence of this error, even a crashed application can cause the hard drive to display this message followed by series of system slowdowns, other applications starts to show weird behavior and download files may transform into corrupt files. Another instance of the occurrence of this error is when an external hard drive is disconnected unethically while data transfer is under way. At times, the hard drive develops the corruption of one of the A.T.A – 100 buses that might not be as serious as MBR corruption, though it allows functioning of the drive but in a troublesome manner.
Also, interruptions through abrupt shutdowns, system or hard drive components going through power jolts may also put an end to the normal functioning of the devices and eventually leads to the corruption of Master Boot Record (MBR). In no such way, the Master Boot Record corruption should be taken lightly because in the worse case of all, it can deprive you the access to your entire system and the data residing on the HD (or an external hard drive). Oops, I must also not forget to mention another worse kind of case where the drive in question may be considered for replacement.
From the user end, verifying the disk and running 'Repair Disk Permissions' could prove to be of some help but the Disk utility may not always be victorious in resolving severe errors, but yes if the damage done on the drive is small then Disk utility can fix it for sure. Another try is to verify the A.T.A controller of the drive & monitoring the connectors of the drive in question followed by a Mac OS X reboot. This could prove to be of little or great help sometimes.
Hard Drive Replacement and other tools of help: Hopefully if the drive is under warranty or comes under 'Apple care Protection Plan', then approaching for the replacement should be on priority. Otherwise, we do have good hard drive repair utilities for Macintosh Hds that claims to repair severe drives (volume) errors including the re-building of directory structure, if damaged. A damaged boot volume can also be put under the scanner of a drive repair utility with the help of a boot DVD which can further help the drive repair its errors.