Hey,
Can you share the powershell cmdlet you are using to reattach VHDX? Is your VHDX used somewhere except iSCSI Target?
The following thread might be helpful: https://social.technet.microsoft.com/Forums/en-US/0913adc7-b775-453a-86c6-130afadda8c2/newiscsiservertarget-returns-with-an-unauthorizedaccessexception?forum=winserverfiles
As an alternative, you can use StarWind VSAN free to create and share iSCSI targets.
https://www.starwindsoftware.com/starwind-virtual-san
Cheers,
Alex Bykovskyi
StarWind Software
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