Finder is unresponsive. I was copying files from one external hard drive to another and Finder froze in the middle. Is it safe to hard shut down my computer?

I am working on a mac mini from 2012 with OS X Yosemite version 10.10.4. I have an external hard drive continuously plugged into the mac. Once a week I plug a second external hard drive into the mac and copy files from one hard drive to the other. The file transfer usually takes one to two hours. I've never had any problems doing this until today. Finder became unresponsive while in the middle of copying files. I cannot eject the hard drives or restart the computer in the normal way. Normally with this sort of problem, I would use the power button on the mac mini to hard shut down. I am hesitant to do this, however, since Finder froze in the middle of transferring files between two hard drives. I am worried that one or both hard drives will be compromised if I hard shut down the computer. It's fine if the file transfer is cancelled, but I do not want to lose any of the original files. Is it safe to hard shut down the computer? If not, does anyone have any ideas on how to safely cancel the transfer and eject the hard drives? Thank you.