What does HRESULT 0x801F0008 (ERROR_FLT_FILTER_NOT_READY) mean?

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The minifilter is registered but has not entered filtering with FltStartFiltering.

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ERROR_FLT_NOT_INITIALIZED	ERROR_FLT_POST_OPERATION_CLEANUP

ERROR_FLT_FILTER_NOT_READY

Registered is not yet active

ERROR_FLT_FILTER_NOT_READY means that the minifilter has a valid Filter Manager registration but has not called FltStartFiltering. Operations that need an attachable instance, including manual attachment, cannot proceed while the driver is in this intermediate state.

FltStartFiltering is not merely a flag change. It tells Filter Manager that the driver can begin receiving volume-attachment requests and I/O callbacks. Those callbacks can arrive before FltStartFiltering returns, so every global table, communication port, lock, and callback dependency used by the driver must already be ready.

What to check

Verify the DriverEntry sequence: initialize driver state, call FltRegisterFilter, complete any required setup, then call FltStartFiltering.
Preserve and inspect the return status from FltStartFiltering; do not continue as though a successful registration alone made the filter attachable.
Check error paths after registration. If setup of a communication port, configuration data, or other prerequisite fails, unregister the filter rather than leaving a half-initialized driver loaded.
Do not use manual attachment as a production workaround. The kernel helper that forces an altitude is documented as a debugging facility, not a replacement for correct instance configuration.

References

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