OK, First of all,
When you drop a clip into a new blank sequence, did FCP prompt you to let FCP change the sequence setting to best match your footage? If YES, then Good.
Now, What hard drive do you files live on?
Do you have a couple sata drives stripped inside the MacPro Tower?
Ext. FireWire drive?
You can’t (well you shouldn’t) have media on the OS boot drive.
You can’t edit off an ext. USB drive.
Your sequence settings must match your clip settings.
Chris Tompkins
Video Atlanta LLC