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I have an external HD mostly used in my Win7. But I transfered some files to it using Ubuntu 12, and now it isn't being reconized in my Windows. I am being prompted to format the HD.
I would reboot into Linux to see if it's a formatting issue or hardware issue... If Linux reads it see what the formatting is (type mount) and go from there.
Changing the partition type requires a partition manager and root privileges. Ubuntu definitely doesn't do this on its own. How to Repair a Damaged Partition Table or MBR might help you.
If you really think that Ubuntu formatted the partition, try installing an ext2 file system driver for Windows - fs-driver.org or sourceforge.net/projects/ext2fsd will do the trick of mounting the drive in Windows. If however the drive is corrupted these will not help you - a Linux boot CD and Dennis' link is your best bet.
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) and go from there.