It may be that when you entered disk management it prompted to "initialize" the xbox drive. This will happen if Windows doesn't recognize the partition table type, such as when you have a Mac or linux disk connected, because these don't use an MBR partition table but just a GPT one. If you definitely didn't format it, you may be able to restore the GPT partition table using a program called TestDisk from CG security to scan the drive, find the partitions, and rewrite the GPT tables.