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:
<key>CFBundleIdentifier</key>
<string>com.apple.driver.AppleSDXC</string>