If you're like me then the one of the first things you do after installing Cygwin is to issue the command mount -c / so that mounts appear directly under / instead of /cygdrive. However the recent upgrade to Cygwin DLL means that this change is no longer persistent. In order to make it permanent you need to edit the file /etc/fstab and add a line
none /  cygdrive binary,posix=0,user 0 0
Further details on the Cygwin site