It returns True for drives which don't exist and raises for paths starting with drives which don't exist.

>>> os.path.exists("C:\\")
True
>>> os.path.ismount("C:\\")
True
>>> os.path.ismount("C:\\doesnotexist")
False
>>> os.path.exists("F:\\")
False
>>> os.path.ismount("F:\\")
True
>>> os.path.ismount("F:\\doesnotexist")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\lazka\AppData\Local\Programs\Python\Python35\lib\ntpath.py", line 290, in ismount
    return path.rstrip(seps) == _getvolumepathname(path).rstrip(seps)
FileNotFoundError: [WinError 2] The system cannot find the file specified: 'F:\\doesnotexist'