External HDD Will Not Unmount

I have a powerbook g4 hdd that i took out of the machine to do maintenance on cus i cant do it on the machine itself and it will not unmount, then when I do get it to unmount after a little bit it will not be able to unmount again...any thoughts...
also what does mount and unmount mean? Do I have a bad hdd and might need a new one?

thanks

You may need to replace the HD already.. The meaning of mount is something like get it onto a partition.. Vice versa for unmount... To run test or repair, most likely u will nid it to be unmount first.. I believe you are able to see it on ur disk utilities...