Hard drives store things with varying forms of ECC. Each sector has an ECC field, allowing it to detect many errors and automatically correct some.

This isn't a replacement for something better, it just allows simple bit errors to be corrected automatically by the drive. The problem is that it's not really obvious when it's happening, and you only notice when it can't fix something. Drives eventually throw a SMART error when it's had to do too many corrections though.

Correctable errors are logged by SMART - see the Raw Read Error Rate, Seek Error Rate, Hardware ECC Recovered, and related attributes.

There are many levels and types of error checking performed in the various layers between the physical signals on the media and the bits that get sent over the drive interface. They are not simple checksums either. Otherwise a modern hard drive would not function...

Most modern media does error checking, and usually error correction as well. This isn't primarily because the media is unreliable, although that is true. The main reason to do ECC is to increase the capacity of the media. Having ECC able to cope with a reasonable number of bit errors allows the bits to be crammed closer together on the media while still being reliable.