I'd like to thank the above authors for their descriptions
of replacement sectors, and for inspiring me to explain it further...
Typically, the manufacturer of a disk allocates a number of replacement
sectors on each track (and a few replacement tracks) in the hopes that
when a bad sector is found, a near by
replacement sector can be used instead to "repair" it.
Despite claims of the above writeups, it is possible and useful for a user
to affect replacement sector use on a modern disk. This can be done
either by triggering the drive's low level format mechanism,
or by using manufacturer supplied repair utilities.
If a disk runs out of replacement sectors, the simplest (although
most drastic) way to fix this is to low level format the disk.
Typically, this involves getting a disk diagnostic utility to give
the low level format command to the disk, which will cause it to scan
itself for bad sectors, and then map the remaining good physical sectors
into a virtual track arrangement, with enough replacement
sectors in each virtual track for future needs. I have several old
IDE disks I have done this to multiple times; each time I low level
format it, the drive gets a little smaller.
All modern hard drives use constant linear density (similar to CLV),
so the mapping between track/sector numbers typically has very
little to do with physical tracks and sectors (other than
continuity) anyway.
The Sun Solaris format utility has provisions to change the number
of reserved replacement sectors and replacement tracks during low level
formatting of SCSI disks. However, I doubt many disks pay attention
to this info. It probably can only really be set at the factory when
the factory low level formats it, or when they write its firmware.
Even so, low level formatting the disk will still redistribute the
replacement sectors.
For modern disks, most disk manufacturers provide diagnostic
utilities (on their web site and/or on a floppy sold with the drive)
which will remap bad sectors that are not automatically remapped.
These utilities typically also accesses the drive's S.M.A.R.T. data,
and uses this as a first pass to guess which sectors to repair.
Use of these utilities is not restricted to advanced users. Most
companies typically recommend you try these utilities before
RMA'ing your drive. IBM's disk warranty web page even includes
instructions "written for new or inexperienced users".
A quick survey of manufacturers found references to repair software
available for download from their web sites:
- IBM
- Drive Fitness Test, Ontrack Data Advisor
- Western Digital
- Data Lifeguard
- Seagate
- SeaTools or Ontrack Data Advisor