There are several software tools available that claim they can “repair” bad sectors on a hard drive. Many people are misled into thinking these utilities are actually fixing the hard drive. To an extent, they are, but in reality what is occurring is something modern hard drives actually do anyways automatically, without the use of specialized software tools. Hard drives have internally programmed maintenance routines that they constantly perform, even while you’re utilizing the hard drive. These programs claiming to “repair” the drive just give the drive an extra “push” to start this automatic, drive-level repair process.

Furthermore, the “repair” is not so much of a repair. You could consider this repair to be the equivalent of a tourniquet. The hard drive is like an arm that has been badly wounded and is bleeding profusely. The tourniquet is the “repair”, which temporarily relieves the underlying issue, but the fact is it’s only a temporary solution to a escalating problem. With hard drives, there is no hospital that can stop them from degrading; we can only hide the degradation temporarily.

So what is really going on when a drive is “repaired”? Reallocation.

A hard drive is constantly performing built-in maintenance routines and when the drive recognizes an unstable sector, the hard drive realizes this, marks the sector bad, then puts it in a reallocation queue.

What is this “reallocation”?
It’s a simple principle: the detected bad sector gets put into a list that says “do not use this sector anymore, because it can’t hold data reliably”. This list is known as the “G-list” or “Grown Defects List”.

Once the bad sector has been removed and put into the G-List, a new, reserved sector is used in its place. This reserved sector is “mapped” as the same LBA (Logical Block Address) as the old, dead sector, but it is in a different physical location on the disk.

This can cause issues once many sectors begin failing because the heads will have to change position to read the new sectors that are located in the reserved areas. This will cause delays in access times and will eventually be noticeable to the end user.

All hard drives contain these reserved sectors and the OS is unable to see or use them. Only the drive knows of their existence and their location on the disk.

There is even more to this story though, as there are two types of bad sectors. There are “physical bad sectors” and “logical bad sectors”.

A physical bad sector is a sector that for some reason can no longer to be magnetically manipulated and used to store data. These are the types of errors that will be mapped to the G-List.

A logical bad sector is a sector that is able to be magnetically manipulated, but for some reason was incorrectly manipulated (maybe power was pulled during a write to that sector or the heads are failing/weak and wrote incorrectly to the sector, etc).

These logical bad sectors can be regenerated (usually by writing fresh, correct data to them) and then reused. But, sometimes logical bad sectors can be incorrectly written to the G-List. This often happens when the heads of a hard drive are failing and are reading good sectors with UNC errors because the read element of the heads is malfunctioning. If this happens, the drive will attempt to queue these sectors for reallocation and, if successful, good sectors can end up in the G-List of the drive. Luckily, if you are in need of your data being recovered, such an issue is reversible. Most data recovery technicians can easily access the G-list of a drive that is held within it’s “service area” and remove false entries, or even clear the list completely.

The last thing that I would like to mention is refurbishing. Refurbishing drives takes a somewhat similar approach as these normal repair programs do: they remap bad sectors. The difference is that good refurb companies have reverse engineered hard drives so that they have the manufacturer-unique commands that tell the drive to reallocate bad sectors to a different list. This list is called the “Primary Defects List” or “P-List”. Sometimes it may be called the “Push-Down Defects List”.

The P-list is typically only written once in a disk’s lifetime. This is typically done at the manufacturing site when the drive is assembled and the servo code is written to the platter surface. The P-list handles reallocation much differently than just replacing the bad sector with a reserved sector. It is a more complicated process that will be discussed in more details in a future post, but, in short, it eliminates the performance issues mentioned that occur when many sectors are replaced with reserved sectors.

I hope this has enlightened some about hard drive repair. In conclusion, there is no real “repair” for a drive. Even the refurbishing technique is a temporary solution. The fact is that when a few sectors degrade magnetically, the degradation usually begins to spread to nearby sectors fairly quickly, and the drive will soon become unusable.