def _volumes(self):
        """ returns a map {volume_id: {serial:, vendor_id:, product_id:, tty:}"""

        # to find all the possible mbed volumes, we look for registry entries
        # under all possible USB tree which have a "BSD Name" that starts with
        # "disk" # (i.e. this is a USB disk), and have a IORegistryEntryName that
        # matches /\cmbed/
        # Once we've found a disk, we can search up for a parent with a valid
        # serial number, and then search down again to find a tty that's part
        # of the same composite device
        # ioreg -a -r -n <usb_controller_name> -l
        usb_controllers = [
            # Leaving these here for reference. The code nominally scanned each controller,
            # but a bug (?) caused it to only pay attention to the last one. That seems to
            # work fine, so the others are commented out.
#             "AppleUSBXHCI",
#             "AppleUSBUHCI",
#             "AppleUSBEHCI",
#             "AppleUSBOHCI",
            "IOUSBHostDevice",
        ]

        cmp_par = "-n"
        # For El Captain we need to list all the instances of (-c) rather than
        # compare names (-n)
        if self.mac_version >= 10.11:
            cmp_par = "-c"

        for usb_controller in usb_controllers:
            ioreg_usb = subprocess.Popen(
                ["ioreg", "-a", "-r", cmp_par, usb_controller, "-l"],
                stdout=subprocess.PIPE,
            )
            usb_tree = _plist_from_popen(ioreg_usb)

        r = {}

        for name, obj in enumerate(usb_tree):
            pruned_obj = _prune(
                obj,
                [
                    "USB Serial Number",
                    "idVendor",
                    "BSD Name",
                    "IORegistryEntryName",
                    "idProduct",
                ],
            )
            r.update(_dfs_usb_info(pruned_obj, []))

        return r
returns a map {volume_id: {serial:, vendor_id:, product_id:, tty:}