I discovered accidentally I was sharing my USB harddrive with the entire network, even though I had sharing turned off. Searching for a solution, I found this:

To view administrative shares, use compmgmt.msc and view Shared Folders>Shares
To turn off administrative sharing for good, edit HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\lanmanserver\parameters to add two new DWORD values, both zero: AutoShareServer and AutoShareWks