Problem moving/copying file to external hard drive

Question: 
I want to copy and/or move a file from my desktop to external hard drive but I can't copy and paste the file to an external drive.
Is there another way to move files?

Answer:
 My guess is the external HD is formatted as NTFS which is a PC format. OS X can read a NTFS drive and copy data on it to your iMac but it cannot write to NTFS. If the drive is going to be only used with a Mac then it needs to be reformatted in Mac OS Extended )Journaled).

This is simple to do and is explained in: http://support.apple.com/kb/HT3509. If you are going to share the external HD with PCs and Macs you can still do this. Here is a MacWorld article that explains how: http://www.macworld.com/article/1165513/how_to_share_an_external_drive_between_a _mac_and_a_pc.html

More discussion here