Modem no longer works after startup/reboot in Mac OS X 10.10 (Yosemite)

MultiTech's modems that use the Exar USB-to-serial chipset recommend using a kernel extension (KEXT, aka driver) for operation. As of Mac OS X 10.10 (Yosemite), Apple requires these drivers to be signed to be able to load the driver in the OS (OS X 10.9 and older didn't enforce signing of KEXTs). Some of MultiTech's drivers were not signed, so they will not load and thus the OS prevents the use of the modem.

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Update to the latest signed version (1.0.4 as of this writing):

https://webfiles.multitech.com/mac-scripts/MTSModemInstallerXR-104.zip

Before installing 1.0.4, remove the old driver by doing the following:
1) In the Finder's Go menu, select Go to Folder....
2) Enter /System/Library/Extensions and go to that folder.
3) In the /System/Library/Extensions folder, find MultiModemUSBXR.kext and select it (click once on it).
4) Move MultiModemUSBXR.kext to the trash (e.g. File-->Move to Trash). You will need to supply the admin username/password to move it to the trash.
5) Now you may install the new driver which will be found in /Library/Extensions (the new location Apple recommends for 3rd party drivers).

Models affected:
MT9234MU-CDC-XR
MT9234ZBA-USB-CDC-XR
MTCBA-C1-U
MTCBA-E1-U
MTCBA-G2-U