Restrictions on external hard drive

I has two external hard drives (USB2). One works find, the other has started giving me an error message “Process terminated due to restrictions on this computer, see you administrator” -> when I click to enter the drive. I can see the folders in Explorer.
If I log in as administrator in safe mode I can get to everything on the drive.
QUESTION: What have I turned on that is causing this error?

Thanks in advance for the help

Rick