I'm no expert but my guess would be that it isn't formatted correctly. I forget which it needs to be, but Windows is looking for either FAT32 or NTFS, and the drive is the opposite.
I know I always used to run into that problem when I would install new internal hard drives. They'd be recognized on the boot cycle, but not show up in Windows until I formatted them correctly.
Note: formatting will wipe the drive, so only try to format it if you don't have anything important on there
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