You learn something new everyday! and today was that the service account "Exchange Trusted Subsystem" needs permission to the folder where the PST's are being imported from / exported too.
Yes, even if you've run EMS as Administrator, the "Exchange Trusted Subsystem" also needs permission to the Share and NTFS file system.
I spent a few minutes trying to import a couple PST's through Exchange Management Shell and noticed the error:
"Unable to open PST file '\\server\pst\user.pst'. Error details: Could not find file '\\server\pst\user.pst' "
So check your permissions, rely on the Effective permissions tab to help you check if you've set them correctly.
If you're still having problems, ensure the account you're using has been granted the "Mailbox import export" role like this:
New-ManagementRoleAssignment –Role “Mailbox Import Export” –User AdminAccount