Depending on your bootloader, you need to set it to go to the correct drive. When it was a primary drive, the drive was considered /dev/hda1. When you switched it to a slave (secondary), it saw it as /dev/hda2, but the bootloader is trying to boot hda1. Simply make it a primary again, boot off it, and change the bootloader settings to the drive it will be when its a slave. Since I don't know the distro, I can't tell you what bootloader you are using. Second, some will allow you to use both GRUB and LILO bootloaders, so it knowing which one would also help.