(Un)mounting disks

Connected to my Mac mini are 2 USB disks of equal size. I want one to be actively used and the other to be a copy of the first. I want that copy to be made once a week and I want the disk to not be accessible for anything but making that copy. So what I'm looking to do is mount the disk, make the copy and then unmount the disk.

I've played around with the command line, but can't seem to figure out how to use the mount and umount commands properly. When 'ejected' from the Finder, I can't seem to come up with the right command to mount the disk again. And when trying to unmount a volume, it keeps telling me it's busy even when there are no open files - see log below.

Any ideas?
This machine runs OS X Leopard 10.5.8 by the way.



[host:~] user% sudo umount /Volumes/backup
umount: unmount(/Volumes/backup): Resource busy
[host:~] user% fuser /Volumes/backup/*
fuser: No match.
[host:~] user% fuser /Volumes/backup/.*
/Volumes/backup/.:
/Volumes/backup/..:
/Volumes/backup/.DS_Store:
/Volumes/backup/.Spotlight-V100:
/Volumes/backup/.Trashes:
/Volumes/backup/.com.apple.timemachine.supported:
/Volumes/backup/.fseventsd: