Целата тук е да се направи Windows 7 да се зарежда по подразбиране.
Как да се разбере какъв е номера на Windows 7 ?

cat /boot/grub/grub.cfg |grep '^menuentry'|awk -F "'" '{print $2}'|grep -v 'recovery'|sort|uniq

Linux Mint 17 MATE 64-bit, 3.11.0-26-generic (/dev/sda4)
Linux Mint 17 MATE 64-bit, 3.13.0-34-generic (/dev/sda4)
Linux Mint 17 MATE 64-bit, 3.5.0-17-generic (/dev/sda4)
Memory test (memtest86+)
Memory test (memtest86+, serial console 115200)
Windows 7 (loader) (on /dev/sda1)

Като се смята от "0" то следва, че "Windows 7 " е номер 5

vim /etc/default/grub

…
GRUB_DEFAULT=5
…

update-grub2

Generating grub configuration file …
Found linux image: /boot/vmlinuz-3.13.0-34-generic
Found initrd image: /boot/initrd.img-3.13.0-34-generic
Found linux image: /boot/vmlinuz-3.11.0-26-generic
Found initrd image: /boot/initrd.img-3.11.0-26-generic
Found linux image: /boot/vmlinuz-3.5.0-17-generic
Found initrd image: /boot/initrd.img-3.5.0-17-generic
Found memtest86+ image: /boot/memtest86+.elf
Found memtest86+ image: /boot/memtest86+.bin
Found Windows 7 (loader) on /dev/sda1
done

reboot

Готово :)