you need to get a kernel that ubuntu can use to boot, and need to configure some files on the ubuntu partition correctly such as watchdog configuration amongst others, however these issues are the same as when installing on the sd-card . I installed debian on my internal card using guide at http://elektranox.org/n900/ and based the information written above about the uboot-part on what's written there at http://elektranox.org/n900/kernel/uboot.html I just made small changes so It can boot from the internal card .