[X4U] DVD won't play; external drive

[netkat at comcast.net]

I have a mini with an external boot drive 
(7200RPM, FireWire, 250 gig, loads of free space).
I can read data DVDs, but I can't play "movie" 
DVDs. Disk Utility is aware when such a disc is 
inserted, but DVD player won't play it, and it 
doesn't show up in the side bar or on the desktop.

if I reboot to the Mini's own internal drive, 
"movie" DVDs do play just fine.

Any ideas on this?

nk