Make sure you restart (turn it off and back on) the phone before trying to insert a new card. Now you shouldn't actually need to mount/unmount it unless that message shows up on the device. Many devices will just work once the card is installed, as long as it is a good brand and preformatted.
So first test to see if the card is working or not. The mount message not withstanding. If you can put items on the card, or see it from the phones file manage program, then it is working. Also if it shows in the phones "Settings" and "Storage" area it is working.
Now if it isn't showing in those, then the card could be the problem, or the port could be damaged. Please remember that while you 'can' insert and remove a SD card from one device or another, it is really not best to do that regularly. Both the port and the card are really not built for regular handling.