Can't boot into OS X

[n8236]

Any reason why I can't boot into OS X while I have my Firewire/USB external hard drive plugged in? I've had my UMBP for over a year and this hasn't happened up until recently.

 

[harry*333]

its probably trying to start up from the harddrive, try restarting
and following these intsructions http://www.macosxhints.com/article.php?story=20080919150504866to set the startupdrive

 

[spinnerlys]

Sometime an SMC reset might help.

Does holding down the Option/Alt key after the startup sound get you a selection screen to choose (from) which (volume) OS to load/boot?