To un-forget the SD card, you can remove it from your Mac and reinsert it after a few seconds.
If your SD card still shows up in Disk Utility, it's best to eject it first before unplugging it. This way, you won't corrupt data on the drive. To safely eject the drive, locate the drive's name, then right-click it and choose Eject. Once the SD card is ejected, you can remove and replug it.
Suppose you still can't access the SD card on your Mac, you can unplug all external peripherals except the power cable, reboot your computer and then connect the SD card to your Mac again.
As a side note, I didn't find the "Forget" button on my Mac. If you mean the "Unmount" button, which will make your SD card vanish from your desktop and Finder when clicked, there's a simpler way to re-access it. Just select the grayed-out partition of your SD card in Disk Utility and click the Mount button to mount it again.
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