How to handle “Data error (cyclic redundancy check)” (system error code 23)

If you get an error like this during image backups:

Error on client occurred: Error while reading from shadow copy device (1). Data error (cyclic redundancy check).

Or like this during a file backup:

Error getting file patch for “x/y/z.file” from CLIENT. Errorcode: ERRORCODES (11)
Remote Error: Reading from file failed. System error code: 23

Or you get the same error while copying/accessing a file in any program (Windows Explorer, etc.) you have damaged sectors on your hard disk. One way to confirm this is to have a look at your disk’s S.M.A.R.T. values (e.g. with SpeedFan; please comment with a better alternative if you have any).

The raw value of “Current pending sector count” should be greater than zero.

As an explanation: You hard disk is designed such that the probability that each sector becomes unreadable is really low. Nevertheless, it is still possible and expected during a hard disk’s lifetime, as such the hard disk is engineered to handle this failure case. The hard disk has a few spare sectors. If a sector becomes damaged, the hard disk replaces this sector with one of those spare sectors. The good case is that the hard disk detects that a sector has problems while still being able to read the data in this sector (e.g. by retrying or using storage redundancy). In this case it can read the data, replace the sector with a spare one, write the data to the spare sector and use the spare sector from then on (S.M.A.R.T. value “Reallocated Sector Count” is increased by one).
If it cannot read the data it has to return a error saying that it cannot read the sector (and increase “Current Pending Sector Count” by one). The next time the sector is written to, it’ll write the data to a spare sector, then replace the damaged sector with the spare one (and decrease “Current Pending Sector Count” by one while increasing “Reallocated Sector Count” by one).
The error being returned by Windows if the hard disk says a sector is unreadable is “Data error (cyclic redundancy check)” (system error code 23).


If you don’t have a copy of the damaged sector you have lost data. You could retry a few times. Put the hard disk in a freezer and then retry or something, but chances are it won’t work.

This doesn’t mean your hard disk is completely broken. It may be an indication that it is going to break completely soon, but you also might have just been unlucky during normal operation.

How to fix it

If the error message includes a file path, replace the file with a backup copy. If you don’t have a backup copy, you can either delete the whole file, or use the scrub tool below to only delete the damaged parts of the file (if it is a movie file or an image it might be mostly okay).

If it is occurring during image backups, the scrub tool has the option to find out which files are damaged. You can then either replace the files with backup copies, delete the files or let the scrub tool delete the damaged parts of the file.

If the damaged files are Windows system files, they might get fixed by running a system integrity check. Open an admin console (WINDOWS + X then “Windows PowerShell (Administrator)”) then enter “sfc /scannow”.

Scrub tool

The UrBackup client (starting with 2.4.x) includes a tool that scans a whole disk, then lists the damaged files, then has the option to delete damaged parts of a file.

  • Download and install UrBackup client from https://www.urbackup.org/download.html#client_windows
  • Go to “C:\Program Files\UrBackup” in Explorer
  • Right-click “scrub_disk.bat” then select “Run as Administrator”
  • Select the volume you want to repair
  • If it finds unreadable sectors it asks if you want to list the damaged files (this might take some time) and if you want to delete those unreadable sectors

RAID calculator/simulator

Go to RAID calculator/simulator

This is a RAID calculator/simulator. Given a desired failure percentage and space overhead, it simulates a few disk arrangements and then outputs the one where the failure percentage is lower or equal to the desired one.

So, you don’t have to read up on when it is best practice to use RAID5 vs. RAID6 or how many disks groups to arrange your RAID50 into.

A few notes:

  • The default disk failure percentage per year is 5%. This is the upper end. Depending on disk model, how much they are used and things like temperature and vibration isolation one would probably set it to more like 1% (See e.g. the statistics here)
  • The Unrecoverable Read Error (URE) rate is 1e14 per default. As mentioned in the Wikipedia article this often also is 1e15. You can get this info from the disk data sheets for example here for Seagate IronWolf.

What can still be improved:

  • Currently it assumes that disk failures are independent (“Independent and identically distributed random variables” from probability theory). I.e. one disk failing doesn’t make it more likely that others will fail. This may not be the case if you put more than one disk of the same model or even production batch into the same RAID group, because those models may all have the same flaws which make the disks die about the same time after identical usage (taking your RAID down with it). So, try to avoid putting identical disks into the same RAID group
  • As one can see it only uses one failure probability per year. A more accurate simulation can probably be had if the failure probability is adjusted for age. Disks fail often early on, then fail less, then start to fail more often when they are older.

Infscape UrBackup Appliance uses this RAID calculator/simulator to automatically arrange the RAID configuration according to the storage failure percentage and overhead you specify. This has a few advantages compared to manually setting up a RAID:

  • It is easier because you don’t have to manually arrange the disks. And it also avoids accidental misconfiguration
  • When a disk fails it can simply recalculate the RAID configuration with one disk less and use this configuration to write new data. After waiting a bit for the disk to be replaced it can also automatically reshape the data into this new configuration
  • When a disk becomes full it can calculate a RAID configuration without the full disk and write more data using this new configuration allowing the RAID to fill disks with different sizes while keeping the overall failure guarantees of the storage

Switched main page and download site to SSL

Letsencrypt is in public beta now. As promised I switched the main page as well as the download site to SSL. The main page redirects to HTTPS and therefore cannot be accessed without SSL anymore.

The Letsencrypt certificate causes the main page to not work with Windows XP anymore. That are 2,6% of all visitors to that page. They are working on a fix for that issue, though. Let me know if there are any other problems.

By always being on a SSL-protected site you can be certain you are getting the official UrBackup binaries even if an attacker is able to modify your network traffic.

Comments should work now

I moved the blog from Sourceforge to a different server, because the spam detection plugins of WordPress do not work on the Sourceforge webspace. The comments should work now. Sorry for the inconvenience.