Attribute ID: 5 (0x05)
Hard drives, supporting this attribute
Samsung, Seagate, IBM (Hitachi), Fujitsu, Maxtor, Western Digital
Reallocated Sectors Count S.M.A.R.T. parameter indicates the count of reallocated sectors (512 bytes). When the hard drive finds a read/write/verification error, it marks this sector as "reallocated" and transfers data to a special reserved area (spare area). This process is also known as remapping and "reallocated" sectors are called remaps. This is why, on a modern hard disks, you will not see "bad blocks" while testing the surface - all bad blocks are hidden in reallocated sectors.
However, the more sectors that are reallocated, the more a sudden decrease (up to 10% and more) can be noticed in the disk read/write speed.
This is a critical parameter. Degradation of this parameter may indicate imminent drive failure. Urgent data backup and hardware replacement is recommended. See also How to Monitor Drive Health Degradation Over the Time in Acronis Drive Monitor.
See also: S.M.A.R.T. Monitoring.