Memory card

SD, microSD, CFexpress and CFast all look interchangeable until a camera refuses to record 4K to a card that fits it perfectly. Two numbers matter. Sustained write speed has to keep up with the codec's bitrate, and the video speed class printed on the label is the manufacturer's guarantee of a minimum sustained rate. Capacity is the number people shop on and the one that matters least. Buy from a retailer you trust, since counterfeit cards with faked capacity are common, and retire any card that has thrown a corruption error rather than trusting it a second time.