How to get rid of unwanted Operating System?

Question

1) Had OS in three different partitions. Reformatted one but all three
still show as choices when booting. How do I eliminate the one
I reformatted.

2) When booting, the top OS is started unless I am present to
choose another. Is there a way to preselect a default OS?

Answer 1

It depends on which bootloader you are using.

Here is an example of setting a default OS to boot using LILO. This is an example /etc/lilo.conf showing two different operating systems. Two only have one, simply delete everything between one of the # Linux bootable partition config begins and # Linux bootable partition config ends lines.

After you have made your changes, don't forget to re-run lilo.

# LILO configuration file
# generated by 'liloconfig'
#
# Start LILO global section
boot="/dev/hda"
message = /boot/boot_message.txt
default = "2.6.18.3"
prompt
timeout="100"
vga="normal"

# Linux bootable partition config begins
image="/boot/vmlinuz"
initrd="/boot/initrd.gz"
root="/dev/hda3"
label="Linux"
read-only
# Linux bootable partition config ends

# Linux bootable partition config begins
image="/boot/vmlinuz-2.6.18.3"
root="/dev/hda3"
label="2.6.18.3"
read-only
# Linux bootable partition config ends

Answer 2

You need to format your whole hard drive.