Problem with kextunload SD card remount
You need to specify the bundle identifier when using the -b argument. 
In this case, the bundle identifier is com.apple.driver.AppleSDXC

sudo kextunload -b com.apple.driver.AppleSDXC

If the same error message is returned, then the kernel extension was not loaded to begin with. 

You can check this information using kextstat | grep -e SDXC

Regardless, to find the Bundle ID:

/System/Library/Extensions/AppleSDXC.kext/Contents/Info.plist the bundle identifier is stored under the BundleID key:

&lt;key&gt;CFBundleIdentifier&lt;/key&gt;
&lt;string&gt;com.apple.driver.AppleSDXC&lt;/string&gt;