Apple Logic Pro 7 (TDM Guide) - Manual (Page 20)

20 Chapter 2 About TDM
Total Number of Audio Tracks
When operating different audio hardware devices simultaneously, the number of
possible playback tracks cannot be determined by merely adding them up. This is
especially true if the connected systems place a strain on the CPU.
The number of tracks may also be reduced if the connected systems access the same
hard disk bus. The impact of this may be significantly lessened (resulting in an increase
in the total track number) by using different hard drives for the various systems. As an
example; DTDM or Core Audio and/or a QuickTime movie reads from an internal drive,
and DAE from an external FireWire or SCSI drive.
Please carefully read and follow the instructions outlined in the Digidesign
compatibility guidelines.