It seems that Grub overwrite your windows bootloader because Grub is not installed on your external drive but on your laptop drive.
Two things to do :
- Restore your Windows bootloader
- Install the Grub bootloader on your external drive so that Grub only appear when you choose to boot on that drive
How to restore your Windows bootloader
- Start Windows with a rescue or boot disk then run
OR Start Ubuntu, install lilo and restore your MBR(Master Boot Record)
sudo apt-get install lilo
sudo lilo -M /dev/sda mbr (assuming that your boot partition is on /dev/sda)
How to install Ubuntu on your external drive without corrupting your windows bootloader
- Start Ubuntu installation process with LiveCD
Select "Specify partitions manually"
Choose partition on your external drive for your datas
Choose the partition to install your Boot loader : here choose the partition on your external drive Caution : default value is
/dev/sda that is your netbook internal drive, you have to change default value !
Go on with your installation the classic way
Using this method, your system will boot on windows unless you plug your external drive and choose it as boot disk (using F12 or other special key at startup).
I use this system with Windows on my laptop and Ubuntu on a usb key and it works fine.