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If you are trying to write from Sentinel to a new (or newly formatted) SD card, Sentinel won't recognize the card until you use the "Clear User Data" function in Sentinel on that card.
Check the box that says "display all drives" when using "Clear User Data" on a new (or newly formatted) card. Select the card reader drive and finish the process.
if you don’t have any luck with that, try another new, or known good SD card, or download and use this utility to format your card.
Watch our video on how to use the SD Memory Card Formatter SD Memory Card Formatter 5.0.3 for SD/SDHC/SDXC/SDUC The SD Memory Card Formatter formats SD Memory Card, SDHC Memory Card, SDXC Memory Card and SDUC Memory Card (respectively SD/SDHC/SDXC/SDUC Cards) complying with the SD File System...
www.sdcard.org
If Sentinel still doesn’t recognize the card reader when you check the “display all drives” box, you may have hardware problems with the card reader or it’s cable.