Boot problem after installing Ubuntu

Hello, yesterday I installed ubuntu 15.10 (replace windows) on my acer laptop and facing the same problem:

"First, as I tu on, there is this blue screen error: "default boot
device missing or boot failed. Insert recovery media and hit any key. Then
select boot manager to choose a new boot device or boot recovery
media". I can press "ok".

Then, it comes a screen with Boot manager, with two choices:
1 unknown device (WD10JPVX-22JC3T0)
2 windows boot manager (WDC WD10JPVX-22JC3T0)

I choose 1.

Finally it comes out Grub and I can start Ubuntu.

Now, Ubuntu runs fine and I am happy, but the booting sequence is a little
aoying and odd, definitely ankward.

Besides, why there is still the windows boot manager if I have removed the original
hard disk with windows? I can't understand, it is like the Windows Boot manager is still hidden in there somewhere."

How can I fix it? Thanks.