Trouble completing export

I am trying to export my Iphoto library of 15 gb to an external hard drive. All goes well for a while but then stalls about 2/3 rd the way through.
Hard drive is formatted for mac 50 gb capacity. I have 1 gb ram. Any ideas?
Also when I try to stop the down load, and close windows, finder tries to relaunch but will not completely and I have to reboot with the on off button. Bob