M-Audio Fast Track USB and Logic Pro X
I wanted to share this tip in case anyone else has the same issue I had.
I attempted to use the Fast Track USB audio interface with Logic Pro X. It had worked in every version of Garageband since version 3, as well as older versions of Logic Express and Logic Pro, so I assumed it would work out of the box with Logic Pro X.
With it plugged in correctly, and my guitar connected to it, I added a new audio track to my project - ensuring the Fast Track was set as the input device, and input 2 was selected (input 1 is the XLR input on the device) - and enabled playback monitoring. There was no sound. Additionally, the button to enable recording was not present for the track. The same was true for every new track I added that used the Fast Track as the input device.
I tried everything: in Logic Pro X > Preferences > Audio > Devices, I toggled between the built-in input and the Fast Track; I restarted the software; I unplugged and plugged in the Fast Track, and I restarted the system. None of these worked. Then I had an idea.
In the Applications > Utilities folder on my Mac, I opened the Audio MIDI Setup app. This opened the Audio Devices window. I clicked on the plus at the bottom left and chose to Create Aggregate Device. I named this device by double-clicking the name on the left, then on the right I checked Built-in Input and Fast Track, taking note of which inputs belonged to the Fast Track in this new device (for me, inputs 3 and 4).
After restarting Logic Pro X, I chose this new aggregate device as my input device, chose input 4 for recording, and suddenly everything worked correctly.
iMac (27-inch Mid 2011), OS X Mavericks (10.9.1)