transferring files to external hard drive
I'm trying to transfer files to an external hard drive to back them up. I first tried using time machine but it wants me to erase all current data on my hd and that is just out of the question. Then I tried opening two windows in finder (one that has the files I want to transfer from my computer and one that has the place where I want them to go on my external hard drive) and dragging them over but it won't let me do this. I'm a new Mac user and oddly enough, this was SO much easier on my PC. I really need these files backed up as it's my entire thesis data. HELP please!
MacBook Pro