A microSD slot on your phone means you are able to increase your phone's storage with ease. Galaxy Note8, S8, and S8+ support microSD cards up to 256GB, while Galaxy S9 and S9+ can support microSD cards up to 400GB.
To insert a microSD card, first eject the SIM tray by using the ejection pin. Insert the pin into the hole on the SIM tray to eject. Then take out the SIM tray, place your microSD card onto the tray, and put it back into the phone.
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* MicroSD card sold separately.