I have a Macbook Pro running Mac OS X 10.6.
I wanted to test how mavericks works on my computer so I downloaded the OS and then I installed on a external USB HDD. Everything work as expected. I can now boot from the USB device with mavericks and I still see my internal HDD (containing Snow Leopard) with all the data untouched.
The problem is that I can't boot anymore from my internal HDD, even if the usb drive is not connected, I can see that the boot is "mavericks-style" and then fails because it doesn't find the drive. It seems that the installation procedure not only installed mavericks on the external drive, but also overwrites the boot sector of the internal HDD?