<DllImport("kernel32.dll", CharSet:=CharSet.Auto, SetLastError:=True)> _
Public Shared Function GetDriveType(ByVal lpRootPathName As String) As Integer
End Function

[DllImport("kernel32.dll", CharSet=CharSet.Auto, SetLastError=true)]
internal static extern int GetDriveType(string drive);

You could use these APIs in managed code as noted above. As an alternative to using unmanaged code, you could consider using the System.Io.DriveInfo class and its DriveType property.

For more info on this class, see http://msdn.microsoft.com/en-us/library/system.io.driveinfo_properties(VS.80).aspx

"To determine whether a drive is a USB-type drive, call SetupDiGetDeviceRegistryProperty and specify the SPDRP_REMOVAL_POLICY property."

This is not safe. I've seen FireWire drives with CM_REMOVAL_POLICY_EXPECT_SURPRISE_REMOVAL and USB drives with CM_REMOVAL_POLICY_EXPECT_NO_REMOVAL (a card reader in a printer).
The better way is to check the BusType for being BusTypeUsb by means of IOCTL_STORAGE_QUERY_PROPERTY.
http://msdn.microsoft.com/en-us/library/ms803642.aspx

Sample:

// szPath without trailing backslash like
//  "\\\\.\\X:"
//  "\\\\\?\\Volume{433619ed-c6ea-11d9-a3b2-806d6172696f}
//  "\\\\.\\PhysicalDrive0"

HANDLE hDevice = CreateFile(szPath, 0, FILE_SHARE_READ | FILE_SHARE_WRITE,
                            NULL, OPEN_EXISTING, 0, NULL);

if ( hDevice != INVALID_HANDLE_VALUE ) {

    DWORD dwOutBytes = 0;           // IOCTL output length
    STORAGE_PROPERTY_QUERY Query;   // input param for query

    // specify the query type
    Query.PropertyId = StorageDeviceProperty;
    Query.QueryType = PropertyStandardQuery;

    char OutBuf[1024] = {0};  // good enough, usually about 100 bytes
    PSTORAGE_DEVICE_DESCRIPTOR pDevDesc = (PSTORAGE_DEVICE_DESCRIPTOR)OutBuf;
    pDevDesc->Size = sizeof(OutBuf);

    // Query using IOCTL_STORAGE_QUERY_PROPERTY 
    BOOL res = DeviceIoControl(hDevice,                     // device handle
                 IOCTL_STORAGE_QUERY_PROPERTY,             // info of device property
                 &Query, sizeof(STORAGE_PROPERTY_QUERY),  // input data buffer
                 pDevDesc, pDevDesc->Size,               // output data buffer
                 &dwOutBytes,                           // out's length
                 (LPOVERLAPPED)NULL);

    CloseHandle(hDevice);

    if ( res ) {
        // here we are
        BusType = pDevDesc->BusType;
    }
}