@Geir Iversen​ Below cluster policy works for me. BTW access to the file system is allowed in single user mode in Unity Catalog. Did you try this access mode?

"data_security_mode": {

  "type": "allowlist",

  "values": [

   "SINGLE_USER",

   "LEGACY_SINGLE_USER",

   "LEGACY_SINGLE_USER_STANDARD"

  ],

  "defaultValue": "SINGLE_USER",

  "hidden": true

 },