Apple Logic Pro 9 (TDM Guide) - Manual (Page 16)

The number of tracks may also be reduced if the connected systems access the same
hard disk bus. This can be mitigated (resulting in an increase in the total track number)
by using different hard drives for the various systems. For example, the audio data for a
Direct TDM or Core Audio device, and a QuickTime movie are read from an internal hard
disk, and audio data for a DAE device is read from an external FireWire hard disk.
16 Chapter 2 About TDM