This runs as SYSTEM, which authenticates to the network as the machine account.
A SYSTEM process reaching a UNC path without credentials gets an access-denied
that Test-Path reports as "not found", so a permissions problem reads as a
missing directory and sends the next person hunting the wrong thing. The
backups registry already carries that warning; the client had not acted on it.
On a write failure the log now names the likely cause, and the two conditions
that make it work: the entry must run under the GE-Enforce cycle, since the
share is mounted only for its duration and all SYSTEM processes share the logon
session that holds the connection, and the share ACL has to admit computer
accounts.
Verified against an unreachable UNC on a bay with a real controller: capture
succeeded, the write failed, the diagnostic was logged and the cycle was
counted as a missed backup.
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