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The camera is capable of many different bitrates, depending on the recording mode you select. Memory cards come in a variety of specifications and speeds that they can support. If your memory card is too slow — then the camera just won't be able to record your chosen recording format onto that card. To use that memory card, you may have to drop your recording format down to a lower-bitrate mode. In general, you can use a Class 6 SD card only for recording AVCHD. You can use a Class 10 card to record AVCHD and 50-megabit FHD only. If you want to record the higher-bitrate modes (FHD 100M, FHD ALL-I 200M, or any version of UHD or 4K) you're going to need a UHS-3 memory card. Don't be fooled by advertising of how many kbits or megabits a card can write (such as "45MB/s" or "300x"); those specifications are not what matters to the camera. The only thing that matters is the minimum sustained write speed, which is specified by either a Class 6 or Class 10 designation, or by the Ultra High Speed designation. UHS-1 cards won't work for the highestbitrate recording modes, you need UHS-3 or MicroP2 cards for those.
Finally, if you've met the other criteria listed above, verify that your memory card works. Sometimes memory cards go bad, so you'll want to test to see if your memory card is working properly. Try putting it in another camera, or in a computer, to see if the card is recognized. On that note, always buy and use the best brand-name cards you can get; it's true that you can usually use cheaper memory cards, but the old adage "you get what you pay for" still applies, so always use the very best memory cards you can afford. And don't be ripped off by counterfeit memory cards! If you're shopping on auction sites or through less-than-reputable resellers, there is a very real prospect of receiving counterfeit ("knockoff") cards. Stick with reputable resellers who are factory-authorized dealers for the memory cards you're shopping for.