Back in the day, I remember that I had to use GRUB2 to boot from a logical partition (given that the os-prober grub2 script was present). I once had 22 linux distros on one 1.5 tb HDD back when I was new to Linux.
PS: Sorry for the necrobump.
I had a usb pendrive fail on me after I did this (I was using Mint, if that makes a difference). Make sure to use a drive that you know is reliable and made by a reputable brand!