It looks like os-prober was automatically wiped-out from the installation due to some odd reason. What this means is that the PostX 0.4 system does not scan for other operation systems during the installation. Remeding this issue should be easy after the installation.
After installation press F3 to open the terminal and paste this in:
sudo apt-get install os-prober && sudo update-grub
Notes for UEFI computers
If your computer is having UEFI (Windows 8 and Windows 10 machines) make sure you enable legacy boot and disable secure boot from Bios. Secure boot remains problematic for Linux bootloaders. In some cases even disabling secure boot does not bring Windows 10 entry to Grub2 menu. Instead you will only get the entry for recovery option.
I recommend that you install PostX to a different hard drive or use it from an usb media – if you have any doubts with UEFI. Example: Your Windows 10 is on /dev/sda and you install and use PostX from /dev/sdb.
If you are very lucky, you might not have any trouble with UEFI after disabling secure boot. But still, I recommend that you use caution with UEFI since it is proven to be highly problematic with some computers and manufacturers.
Important update for UEFI. I have now ran some tests with Windows 10 UEFI and I recommend that you either boot PostX Gnu/Linux from an usb media and use it from there or then install it to a separate media alltogether. As an example, I have Windows 10 installed on my standard hard drive and then I have a small separate external usb ssd drive, which has my Linux in it (Grub and boot partitions included). The previous leads to the scenario where I can set my Bios to a legacy boot and disable secure boot but still use either Windows 10 or Linux based upon my choice during the boot. By default my computer tries to boot Windows 10 but if I press F9 key and choose to boot from my external usb ssd then I will boot to Linux instead.
On Linux I can then install os-prober and update-grub and see other operation systems installed – except my Windows 10 because of the UEFI maddness.