Upon inserting an SD card into a reader, Windows may occasionally pause and claim that the card needs to be formatted. But Windows may be wrong. All it takes for an SD card to seem unreadable is for the initial read of the file table to be slightly interrupted. Formatting a drive removes all data from the device and isn't always necessary. When presented with this message, try a few workarounds before formatting.
On occasion, a card will begin reading before it's fully inserted. As a result, the initial negotiation that gives Windows a look at the file table can, if interrupted, fool the computer into thinking that the file table doesn't exist. If Windows fails to detect the file table, it will assume that one has not been established and prompt you to format. Remove and carefully reinsert the card. This often remedies the issue and gives you access to the functioning card. The low-power state of a computer, such as when in screen saver or sleep mode, can sometimes enable the false reading.
Sometimes there are small discrepancies between the card reader and the speed of the SD card that make proper communication difficult. To test this, try the card in another reader on another computer, if possible. If the SD card works properly when placed into your smartphone or camera, try returning the card to that device and instead connecting that device via USB if possible. Sometimes the issue occurs when a high-capacity, newer SD card is inserted into a significantly older card reader.
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