When Apple shipped Xcode 7, it decided to ship it only with the OS X 10.11 SDK. Unfortunately, this has since put developers of USB kernel extensions between a rock and a hard place.
OS X 10.11 ships with a new USB stack and also took care of backward compatibility for both the kernel and user space APIs:
OS X v10.11 Changes
This article describes changes to USB in OS X v10.11.
The USB stack is completely redesigned to increase stability and performance compared to Mac OS 10.10. Applications and third-party drivers for vendor-specific USB devices built with 10.10 SDK require no modification.
Applications that use IOUSBLib APIs should continue to work with no modification. IOKit drivers that use IOUSBDevice and IOUSBInterface APIs should continue to load and be functional with no regressions.
If you are developing a new IOKit driver that uses IOKIT USB APIs, please refer to the OS X v10.11 SDK for new API and classes such as IOUSBHostDevice and IOUSBHostInterface. New applications developed using the OS X v10.11 SDK should continue to use the IOUSBLib APIs.
Different classes of USB devices should not exhibit any regression in functionality compared to the OS X v10.10 release.
In essence, that means you can still write a USB kernel extension targeting older OS X releases and it will continue to just work™ under OS X 10.11, too.
The OS X 10.11 SDK, however, lacks the needed header files for the old USB stack. And that’s why the OS X 10.9 SDK is still needed if you want to build these.
Up until Xcode 7.2.1, this was as easy as copying the folder MacOSX10.9.sdk folder from Xcode 6.4 over into the Xcode 7.x application bundle.
Not so in Xcode 7.3 – apparently because its MacOSX.platform contains a new key, specifically telling Xcode to ignore any SDKs prior to OS X 10.11.
Here’s what I had to do to get Xcode 7.3 to work with the OS X 10.9 SDK:
Quit Xcode 7.3.
and remove these two lines:<key>MinimumSDKVersion</key>