There is a problem with OS X 10.10 with some GPS devices such as the iBlue 747, the bluemax 4044 and some Qstarz devices. In case you successfully used your GPS logger with OS X 10.9.5 and it no longer works with 10.10 then please open the preferences in myTracks. Switch to GPS Device and check the row Device. If the value starts with usbmodem then you are affected by this problem.
The affected GPS loggers use a standard USB protocol (USB-CDC-ACM), that is designed for classical modems and cellular modems. The marketing devision of Apple has decided that the driver should only work with real communication devices. Therefore the driver is no longer loaded when a GPS device is connected.
Therefore I developed an own driver based on the one of OS X 10.9.5. Please download and install the driver, reboot your Mac and everything should be working again. If not, please try to run the GPS Logger Assistant again (myTracks->Preferences->GPS Device).
Important: Some time ago I provided a patch for the existing driver instead of this standalone driver. In case you already installed the patch please start the helper application (USB-CDC-ACM Patcher) and disable the patch. Afterwards please install the new driver.