hard drive - SMART extended self-test passed without error but SMART status check indicated failure imminent

My PC failed to boot today. On starting up, the machine made consistent noise as if it tried to access the hard drive, and soon the console showed 'Please back up your hard disk and have it replaced!'

So I went in bios and did a SMART extended self-test.

The result is as follows:

Test Finished: Passed

Self-Test Status Value: 0 (completed without error)

Then I carried out a SMART status check, but this time I am getting 'Hard disk failure is imminent. Please back up your hard disk and have it replaced!'

So could someone please explain to me why SMART extended self-test passed without error but SMART status check indicated failure imminent?

Answer

SMART self-tests will do the following:

Short

Checks the electrical and mechanical performance as well as the read
performance of the disk. Electrical tests might include a test of
buffer RAM, a read/write circuitry test, or a test of the read/write
head elements. Mechanical test includes seeking and servo on data
tracks. Scans small parts of the drive's surface (area is
vendor-specific and there is a time limit on the test). Checks the
list of pending sectors that may have read errors, and it usually
takes under two minutes.

Long/extended

A longer and more thorough version of the short self-test, scanning
the entire disk surface with no time limit. This test usually takes
several hours, depending on the read/write speed of the drive and its
size.

The following SMART attributes indicate a potential imminent failure:

• Reallocated Sectors Count


• Spin Retry Count


• SATA Downshift Error Count or Runtime Bad Block


• End-to-End error / IOEDC


• Reported Uncorrectable Errors


• Command Timeout


• Reallocation Event Count


• Current Pending Sector Count


• Uncorrectable Sector Count or Offline Uncorrectable or Off-Line Scan Uncorrectable Sector Count


• Soft Read Error Rate or TA Counter Detected


• Drive Life Protection Status

The likelihood here is that your drive has passed an extended test, which tests the disk surface and electrical and mechanical performance of the drive, however it could have failed for any of the above issues which may not be a part of the test. These attributes are mostly marked discreetly all the while the drive is in use.

It's possible the drive has failed at some point in use, but the issue could have either not been included in the extended test, or is of an intermittent nature that didn't show during the test.

If you can get the machine booting, using a utility that can show the SMART attribute statuses would give you more information and hopefully narrow down the cause. Either way, it's very often the case that if SMART has displayed a failure, the drive is on it's way out and it will usually be a sign of problems to come. In this case, it seems the drive is a likely cause to your issue.

Further reading - S.M.A.R.T. - Wikipedia