[X4U] External Drive Mini: DVDs won't play

[netkat at comcast.net]

I have a Mac Mini whose boot drive is an external 
firewire 7200RPM 250GB Drive.
When I boot to that drive, I can't play Movie 
DVDs, yet I can read Data DVDs..

When I boot to the Mini's internal Drive, I CAN 
play movie DVDs

anyone know what would explain this? also, I 
can't do a "Repair Disk" on that external drive, 
tho Verify is available to me.

thanks!

nk