When recording 720p/24 to a P2 card you get 1GB minute per but when I use FCP 5.0.4 and record using "Capture Now" the file size is about 335MB per minute. (My "Capture Now" setting is DVCPRO HD - 720p/24.) That's a third of the file size. Why is this? Is the footage that I'm recording in FCP compressed even more because it is a Quicktime file? What is this doing to my image?

Also I notice the data rate in FCP shows that the P2 footage has a data rate of 14.1MB/sec and the footage captured via "Capture Now" has a data rate of 5.7MB/sec. Does this matter?

What am I lossing by going directly to a hard drive and not using the P2 cards?

You're losing nothing, going direct to the hard drive means that Apple is stripping out the duplicate frames and only recording the 24 active frames.

You can do the same thing on the P2 card by choosing 720/24pN mode (not 720/24P mode).