@Brad Williamson - Thanks for the question and using MS Q&A platform.
According to the offical documentation, the latest SHIR has a security fix that has been changed and does not allow access to local files by default.
You must run
.\dmgcmd.exe -DisableLocalFolderPathValidation
to disable the setting for more details, refer to Set up an existing self-hosted IR via local PowerShell
For more details, refer to Access denied to server in File System Linked Service in Data Factory.
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