At startup MacPro, "alt-key" does not work

[macpro20]

My MacPro have OSX 10.6.6 and have two OSX's each on an apart Hard Disk.
One system I use for the regular things and the other system I use only for video-editing.
Always could I choise for the system I want on that moment, by hold pressing the "alt-key" on startup of the Mac pro, till an menu appears where I could click on the HD I want to use.
Yesterday I have got an Update for OSX and have allowed to install that update.
But now works the "alt-key" not more, after a while pressing the alt-key the Mac do nothing, everything stays black.
Have anyone an idea how this comes and what to do on it to resolve the problem?

 

[gullySn0wCat]

System Preferences > Startup Disk then choose which HDD you want to boot off.

If you want to choose by pressing alt, try a PRAM and SMC reset, they might help.

 

[triceretops]

PRAM reset should fix it

I had the same problem and resetting pram solved it.

 

[Macsonic]

I also encountered this situation when the option key does not work and found out the keyboard was defective and had an electrical ground. Those with the thread starter might be a dfferent situation.