No operating system found
Is a message from the BIOS. It has not even tried to read the boot drive. Your BIOS is probably checking the bootable flags on the partitions on the drive.
Many BIOSes don't bother, others need exactly one partition to be flagged bootable.
Look with fdisk -l
Device Boot Start End Blocks Id System
/dev/sda1 * 1 5 40131 fd Linux raid autodetect
/dev/sda2 6 70 522112+ 82 Linux swap / Solaris
/dev/sda4 71 121601 976197757+ 5 Extended
/dev/sda5 71 724 5253223+ fd Linux raid autodetect
/dev/sda6 725 121601 970944471 fd Linux raid autodetect
If you don't have it set (or have too many set), use fdisk to fix it. Its harmless to your data