What does Windows error code 149 (ERROR_IS_SUBST_TARGET) mean?

ERROR_IS_SUBST_TARGET

What ERROR_IS_SUBST_TARGET means

ERROR_IS_SUBST_TARGET is Win32 system error code associated with the documented message: “An attempt was made to join or substitute a drive for which a directory on the drive is the target of a previous substitute.” The significant condition is that the requested drive-substitution topology would reuse a directory that already participates as a SUBST target.

SUBST mappings are scoped to a logon context and form a small namespace graph. Diagnosing only the visible drive letter can miss the underlying target path and an earlier mapping created by another script.

Likely causes

• the destination directory is already the target of a previous SUBST mapping
• a startup script recreates mappings in a different order than expected
• a stale session-level mapping remains after an installer or logon script exits

Troubleshooting steps

1. run the mapping query in the same logon session that receives the failure
2. resolve both drive letters to their final canonical paths
3. remove only the conflicting mapping and recreate the desired topology in dependency order

Code-specific investigation notes

SUBST does not create a real disk volume; it creates a drive-letter alias for a directory. Consequently, a repair that only changes the visible letter can leave the underlying target cycle untouched.

Because mappings are often established by logon scripts, compare an interactive administrator shell with the exact user session and elevation level used by the failing program.

When removing a mapping, first verify that no service, scheduled task, or installer still uses the substituted letter as a stable path. Replacing it mid-operation can redirect writes to an unintended directory.

Example

A logon script creates several substituted drives in sequence. One target is already part of an earlier mapping, so the later command fails with it.

Difference from related errors

ERROR_IS_SUBST_PATH reports that a path itself belongs to a substituted drive; it instead rejects a new mapping because the proposed target is already used by another substitution.

References

• Microsoft: System error codes 0–499