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Users frequently see that they are getting UDMA CRC errors (SMART attribute 199) reported for some drives.

A CRC error indicates that the drive has noted that it has not correctly received the data from the host, and typically indicates a connection issue of some sort. This will normally trigger Unraid to attempt a recovery by resending the data. If this resend succeeds then Unraid will continue as normal, but the retry will have the end effect of slowing down the perceived write speed due to the delay caused by the retry. These retry attempts will show up as entries in the syslog.

If the recovery attempt fails then after several retries Unraid will decide it has a Read error on the drive. If the array is parity protected this will then trigger Unraid to attempt to rewrite the problem sector with the correct data based on the parity data and the corresponding sector on all the 'good' data drives. If this write succeeds then Unraid will continue as normal, but if the write fails Unraid will disable the drive and stop using it. This disabled state is indicated in the GUI by the fact that the drive is flagged with a red 'x'.

Possible causes of such errors are:

  • Badly seated SATA cables. The SATA connector is not a very robust connector and can easily work slightly loose due to vibration or tension on the cables. It is also a good idea to avoid 'bundling' the cables together in an attempt to tidy up the cabling as this can result in cross-talk interference between the cables.
  • Bad SATA cables.
  • Insufficient power to the drive. Quite frequently associated with splitter cables on the power lines, but also possible if the power supply is insufficient to power all drives simultaneously so that power dips occur.
  • Badly seated disk controller
  • Genuine disk problem (least likely but not unknown).

The first guidance that is always given in the forum when a user reports CRC errors is to first carefully check the SATA and Power cabling to the drives as this is by far the commonest cause of such issues.

Points to note are:

  • Occasional CRC errors are not really a cause for concern but if they are happening regularly or in large numbers then this definitely needs looking into to determine the cause and fix the issue.
  • It is not unusual for the Current pending sector count (SMART attribute ) 197 to also be incremented when getting a lot of CRC errors. This is more of a cause for concern as it means these sectors may not be read reliably if another disk fails and a recovery action needs to be performed as error-free recovery requires all 'non-failed' drives to be read without error.
  • The CRC count is stored internally in the drive's SMART information and never resets to zero - it can only ever increase.
  • A CRC error will result in the Dashboard page indicating a SMART-related issue has happened by an orange 'thumbs-down' symbol against the drive.
  • The user can acknowledge they have seen such an error by clicking on the orange symbol and choosing the 'Acknowledge' option from the resulting menu that is displayed. This will result in the icon turning green again and Unraid will only prompt you again if the value of the CRC count increases.