ERROR_NOT_READY - 21 (0x15)
The device is not ready.
Updated: Feb 21, 2026
Technical Meaning
The ERROR_NOT_READY error code indicates that the requested operation cannot be performed because the device or resource is not currently in a state to support it. This typically means that the device has not yet completed its initialization process, or it is temporarily unavailable due to system constraints.
Error Details
This error can occur in various contexts where a device or resource needs to be ready before an operation can proceed. It is a generic error code used by the Windows operating system and APIs to indicate that the required state of the device has not been achieved yet.
Usage Context
ERROR_NOT_READY can appear when attempting to access hardware devices, such as disks, network interfaces, or other peripheral components. It may also occur in scenarios where a resource needs to be initialized before it can be used by an application.
Developer Interpretation
When encountering ERROR_NOT_READY, developers should consider the following:
- Ensure that all necessary initialization steps have been completed for the device or resource.
- Verify that the operation is being performed within the correct context, such as ensuring that a disk has finished spinning up before attempting to read from it.
- Check if there are any system-level constraints that might prevent the device from becoming ready, such as power management settings or driver issues.
Related Errors
ERROR_DEVICE_NOT_CONNECTED(219)ERROR_DEVICE_POWERED_OFF(537)ERROR_DEVICE_DOES_NOT_EXIST(1168)
FAQ
Q: What does the ERROR_NOT_READY error mean?
A: The device or resource is not currently ready to perform the requested operation.
Q: How can I resolve this issue?
A: Ensure that the device has completed its initialization process and that it is in a state where it can support the requested operation. Check for any system constraints or power management issues.
Summary
ERROR_NOT_READY is a generic error code used by Windows to indicate that a device or resource is not ready for an operation. Developers should ensure proper initialization of devices and verify the operational context before performing operations that require the device to be in a specific state.