diff --git a/playbook/shopfloor-setup/common/scripts/Backup-PartMarker.ps1 b/playbook/shopfloor-setup/common/scripts/Backup-PartMarker.ps1
index 95f139a..957f3cd 100644
--- a/playbook/shopfloor-setup/common/scripts/Backup-PartMarker.ps1
+++ b/playbook/shopfloor-setup/common/scripts/Backup-PartMarker.ps1
@@ -390,6 +390,26 @@ try {
     # exactly the kind of quiet breakage this count exists to surface.
     Add-ShopdbBackupSkip -Kind $KIND -Reason 'could not write to the share'
     Write-ShopdbBackupLog -Kind $KIND -Message "Could not write $snapshotdir : $_"
+
+    # Name the likely cause, because the symptom lies. This runs as SYSTEM,
+    # which authenticates to the network as the MACHINE account, and a SYSTEM
+    # process reaching a UNC path with no credentials gets an access-denied
+    # that Test-Path reports as "not found" - so a permissions problem looks
+    # like a missing directory and sends the next person hunting the wrong
+    # thing entirely.
+    #
+    # The share is mounted only during the GE-Enforce cycle, and all SYSTEM
+    # processes share one logon session, so a manifest entry inherits that
+    # connection. A scheduled task would not - it would run outside the cycle
+    # and find nothing there.
+    if (-not (Test-Path -LiteralPath $ShareRoot)) {
+        Write-ShopdbBackupLog -Kind $KIND -Message (
+            "$ShareRoot is not reachable from this process. Running as SYSTEM, " +
+            'so this is usually the machine account lacking rights on the share ' +
+            'rather than a missing path - Test-Path cannot tell the two apart. ' +
+            'Confirm this runs as a GE-Enforce manifest entry (the share is ' +
+            'mounted only during the cycle) and that the ACL admits computer accounts.')
+    }
     exit 0
 }