Soeedi100
At the onset, I am strictly an Elements Windows user, but I try to find common troubleshooting ground between Premiere Elements Windows and Mac.
With that thought in mind, my first question would be "How is the external hard drive formatted?" There would be other questions after that. But, please check out the following article on Mac and External Hard Drives and then let us know if there is any information in it that applies to your situation.
How to set up an external hard drive for use with OS X - CNET
Thanks.
ATR