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I have an external USB-drive which is giving me the following output on running the command

$ smartctl /dev/sdb -H

on it:

SMART Status not supported: Incomplete response, ATA output registers missing
SMART overall-health self-assessment test result: PASSED 
Warning: This result is based on an Attribute check. 

Could you elaborate if this is something to worry about or if it is just a wrong setting? Generally, what is the meaning of the health status in simplified form?

Maybe as a relevant aside: The short and long tests finish without issues.

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  • Usually, external USB-drives are really SATA-drives with a USB-to-SATA-bridge which is part of the enclosure. This bridge may not pass on all SMART commands, and/or the harddrive itself may not support all SMART commands. It looks like smartctl is using some workaround in this situation, which may not be as reliable as the "right" method, but I'd have to look at the code to be sure. If the attributes you get with -A look good, and you can run self-tests and they pass, no reason to worry.
    – dirkt
    Aug 7, 2017 at 21:49

2 Answers 2

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I haven't seen this kind of warning you've got, yet. But apparently it means that smartctl only evaluated the attribute table (see below) because there is no further information from SMART explicitly about the health which is typically a part of the ATA protocol. The response overall is considered not reliable in this case by the author of smartmontools. Drives attached directly to a SATA controller work better with SMART from what I've seen so far.

As concerns the attribute table, when you take a look at a SMART attribute output with smartctl -A /dev/XXX, you'll see three columns VALUE, WORST and THRESH. Here a part of such an output:

ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  1 Raw_Read_Error_Rate     0x002f   200   200   051    Pre-fail  Always       -       0
  3 Spin_Up_Time            0x0027   189   182   021    Pre-fail  Always       -       5508
  4 Start_Stop_Count        0x0032   100   100   000    Old_age   Always       -       18

The first column VALUE tells you the current value of the attribute. The WORST column tells you the worst (typically lowest) value SMART has ever seen. The THRESH column tells you what the vendors considers as lowest possible value considered as healthy.

If the WORST column shows values below THRESH in same row, the drive is considered as not healthy. It also implies that VALUE has been seen below THRESH, of course. You can also see that only the attributes of type Pre-fail matter when evaluating health. Other thresholds are simply set to 0 and their attributes cannot fail.

This table is all that smartctl used for the analysis of the drive's health. And it is not really the correct way to do it right.

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  • 2
    What is the correct way to do it right? Aug 9, 2017 at 14:41
  • Last sentence is probably not very clear. I was referring to the restriction in the protocol that affects the decision of smartctl if a drive is healthy. There is nothing you can do except to attach the drive to a SATA controller or someone decides to fix the controller protocol over USB, which is rather unlikely. Aug 9, 2017 at 14:50
  • Do you know what other information smartctl would get from a SATA controller in this case? I'm in the same situations as OP and I'm trying to figure out how much I can rely on the overall-health test in the given situation.
    – Rotareti
    Jan 7, 2020 at 17:52
  • You should be able to get the regular SMART attributes listing and overall health information after attaching the drive to a SATA controller. These are still product specific and I cannot tell you how they look like and what is included. There are some samples of smartctl -a device output on the web. Jan 12, 2020 at 9:07
  • You can rely on the output usually, if it looks bad. From my experience the drive's guarantee service department never rejected my claims when I specified a faulty SMART attribute as reason for replacement. Jan 12, 2020 at 9:14
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I just found this thread whilst looking for something else, but as a possibly interesting additional piece of data, I have two identical drives, bought on the same day from the same supplier and so probably from the same batch. One of them shows the message reported above, the other doesn't.

$ sudo smartctl -H -d sat /dev/sda smartctl 6.6 2017-11-05 r4594 [aarch64-linux-5.10.17-v8+] (local build) Copyright (C) 2002-17, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF READ SMART DATA SECTION === SMART overall-health self-assessment test result: PASSED

$ sudo smartctl -H -d sat /dev/sdb smartctl 6.6 2017-11-05 r4594 [aarch64-linux-5.10.17-v8+] (local build) Copyright (C) 2002-17, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF READ SMART DATA SECTION === SMART Status not supported: Incomplete response, ATA output registers missing SMART overall-health self-assessment test result: PASSED Warning: This result is based on an Attribute check.

The selftest log outputs (smartctl -l selftest -d sat /dev/sdX) both show tests completed without error. They are in a RAID 1 configuration so I'm not overly worried other than to have SMART set to mail me if it detects an actual problem.

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