The instructions on this site were invaluable to help me fix a USB drive that had multiple partitions and was no longer usable in Windows.
If your USB flash drive, SD card, or another drive isn’t working quite right, “cleaning” the drive and removing its partitions is one possible solution. This can fix problems with a drive that can’t be formatted or one that shows the wrong capacity.
Source: How to “Clean” a Flash Drive, SD Card, or Internal Drive to Fix Partition and Capacity Problems