Check it via the disk management snap-in:
Right-click My computer, select manage
Click Disk Management and screenie that for us, may shed a little light -
I would agree most likely the External Drive.
The biggest oddity is that a "Raw" partition to Windows is usually an initialized disk with no drive letter or file system. Sometimes drives that have been configured in a RAID config will show up like this as well (For instance, if you pulled drive 0 from an array and attached it to another System to view while troubleshooting)
It could be a Factory set partition on the Ex-HDD which holds migration tools or some shit
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