You're right to counsel against them. Most My Book's won't be bootable on Power PC Macs, but my model is one of the rare exceptions...
http://wdc.custhelp.com/cgi-bin/wdc....p?p_faqid=1787
"WD external hard drives that are Mac bootable and how to format them to install an operating system..."
"...The following external hard drives should be bootable on Power PC based Mac computers systems through FireWire (1394a/b):
WD My Book Premium Edition
WD My Book Pro Edition"