[kovidgoyal]

I thought of another possible point of failure. Once again try running calibre-debug and at the green prompt:

import wmi
from calibre.devices.prs505.driver import PRS505
c = wmi.WMI()
for drive in c.Win32_DiskDrive():
    print str(drive.PNPDeviceID)
    if PRS505.is_device(str(drive.PNPDeviceID)):
        print drive.Partitions
        if drive.Partitions == 0: continue
        partitions = drive.associators("Win32_DiskDriveToDiskPartition")
        print partitions