You need to take the user to the device's built-in Settings. I think this will work.
Intent i = new Intent(android.provider.Settings.ACTION_MEMORY_CARD_SETTINGS);
startActivity(i);
Unmounting the SD card is one of those actions which could be used maliciously if it wasn't under full user control. If it could be done purely in software (without user intervention) then code could disrupt other apps running on the device.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…