InfiniteMac OSx86 - external harddrive usb

i have a triple boot kalyway 10.5.2, vista and ultimate edition.
my system is acer aspire m1100
dual core amd 64 4400+
4gig ddr2 ram
nvidia 8400gs 512mb graphics card
one 320 gig sata hd (vista and ultimate)
one 80 gig ide hd (osx kalyway)
and dual monitors one samsung 22in wide and benq 19in wide.

for vista and ultimate i am using grub as a boot loader.
for osx i am using acers boot option to boot to the ide drive which then boots darwin.

I recently bought a terrabyte hd external usb works great on vista and ultimate but if i boot into osx86 with the external on and plugeed in, osx freezes in the boot process if i unplug it it boots fine. then i can plug it back in after and all is well i can even use the drive. Any ideas.