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ERROR_IS_SUBSTED
What ERROR_IS_SUBSTED means
ERROR_IS_SUBSTED is a Win32 system result whose documented message is: “An attempt was made to use a JOIN or SUBST command on a drive that has already been substituted.” In practical troubleshooting, it belongs to legacy SUBST drive-letter mappings where the selected drive letter already redirects to a directory.
Typical causes
- a logon script executes the same SUBST command repeatedly
- another program has claimed the intended drive letter
- stale mapping state remains in the session
How to investigate the result
- run a mapping inventory in the affected interactive session
- Compare the existing destination with the requested destination
- Check scheduled tasks and logon scripts for competing SUBST commands
Developer guidance
Treat an identical mapping as an idempotent result and fail visibly on a conflicting destination. Do not overwrite a drive letter owned by another component.
Administrator and support guidance
Choose a dedicated drive letter and remove duplicate mapping scripts. Remember that SUBST state can be session-specific and may differ between administrator and service contexts.
Example
Two developer tools both attempt to assign drive S: with SUBST, and the second tool receives this result.
How it differs from related results
It means the drive is already a SUBST mapping, not that a path merely lies beneath a substituted drive.
References
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