Someone else reported a similar problem. Perhaps her answer will help you.
"Bill, Thanks for your reply and suggestions. It was not the HD but windows 7 deciding I did not have permission, I then changed the setting and gave myself full permission and all works well. My first reply to a post, hope this is the way it is done. Kind Regards"
Her original query was: "I have just purchased a portable mass storage HD, thinking this would be ideal to copy my Family tree onto from my desktop, when leaving home to take with me. That way I would not need to load it all on to my laptop.
"I copied the paf file plugged it into laptop only to find it will only open in Read Only mode, no editing. Boxes for read only in properties are unchecked... Now if I copy it to my laptop I am able to edit it.
"Any suggestions on changing this greatly appreciated.