What does Windows error code 21 (ERROR_NOT_READY) mean?

 
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ERROR_BAD_UNIT ERROR_BAD_COMMAND

ERROR_NOT_READY

The device cannot service the request yet

ERROR_NOT_READY indicates that Windows located the device but its current state prevents the requested I/O. Typical examples include an empty removable drive, a spinning-up disk, a disconnected backing device, a tape unit awaiting media, or hardware that has not completed initialization.

Useful diagnostics

  • Verify media presence, cabling, power, and the device status shown by Plug and Play.
  • Check whether the device is transitioning after resume, reset, mount, or hot-plug.
  • Record the exact operation and device path instead of assuming a filesystem problem.
  • Review storage, USB, or controller event logs for lower-level errors.

Retry policy

A bounded retry with delay can be appropriate when readiness is expected to change, such as immediately after insertion or startup. Do not retry indefinitely. If the device requires user action, present that action clearly; if readiness never arrives, surface the underlying hardware or driver condition.

References


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