DESTINATION DRIVE CONNECTION PROBLEM

I wonder if anyone can help.  I have just purchased a seagate 2TB external hard drive to back up my laptop data.  It worked first time.  Now when I try to back up it goes for a little while then an error message comes up that the destination external hard drive is not connected - this is when it is still recognised in the my computer window and I haven't moved it.  Can anywone help me get over this?