DiskScan is a Unix/Linux tool to scan a block device and check if there are unreadable sectors, in addition it uses read latency times as an assessment for a near failure as sectors that are problematic to read usually entail many retries. This can be used to assess the state of the disk and maybe decide on a replacement in advance to its imminent failure. The disk self test may or may not pick up on such clues depending on the disk vendor decision making logic.