Error handling in Hard disk in computer architecture:

Modern drives also make widespread use of Error Correcting Codes (ECCs), specifically Reed-Solomon error correction. These techniques store additional bits for each block of data that are resolute by mathematical formulas. The extra bits permit many errors to be fixed. Whereas these additional bits occupy space on the hard drive, they permit higher recording densities to be employed, resulting in much higher storage capacity for user data. In the year 2009, in the newest drives, low-density parity-check codes (LDPC) are supplanting Reed-Solomon. LDPC codes are able performance near to the Shannon Limit and therefore allow for the highest storage density available.

Typical hard drives effort to "remap" the data in a physical sector that will be bad to a spare physical sector-hopefully whilst the number of errors in that bad sector is still little enough that the E CC can fully recover the data without loss.