os.path.splitdrive(path) は,path を (drive, tail) に分割.drive はドライブ名です.tail はドライブ以降です.
import os.path
path = '../../../../../../test.txt'
abs_path = os.path.abspath(path)
drive, tail = os.path.splitdrive(abs_path)
print('drive: {0:s} tail: {1:s}'.format(drive, tail))
drive: Z: tail: \www\test.txt
drive: tail: /home/yamamoto/www_server/yamamoto/www/test.txt