put the tower drive into target mode conect it to your powerbook, restart the powerbook holding down the option key , choose the hard drive from your tower and boot from that,
Now your powerbook will be booted from your towers hard drive.
it would not be a good idea to keep your backups out of a inclosure, there would be nothing to protect the drive, especialy if it gets tuched bysome one with even a tiny amount of static electricity, that could posibly distroy the electronic cercutry on the drive.