data_security_mode": "NONE":  This is a non-Unity Catalog Cluster. No Governance enforced.

"data_security_mode": "USER_ISOLATION": This is a UC Shared Compute cluster that has certain limitations when accessing Low-Level APIs, RDDs, and dbfs/data bricks folders. 

If the .pem files are copied under /Workspace/Shared or /Volumes you should be able to access them via

/Workspace/Shared/file.pem
/Volumes/path/file.pem 

Please make sure READ access to these folders is available.