Hello freebsd gurus ... I have what I think is probably an easy
question for you: how can I make a backup disk that has /, /usr, /
tmp, /var etc. partitions so that it is bootable?

Here's the situation: I come for a Linux background but inherited
admin of a FreeBSD system (still 6.1). This system has /, /tmp, /var
on one disk, and /usr on another disk. There was no backup system.

My first move was to institute some sort of backup, and I chose to use
one that I find convenient on Linux: have a backup disk that is
rsync'd periodically with the main disks, and make it bootable so that
if anything happens to the main disks, I can just swap cables and boot
off the backup.

The first part is done: I installed one disk big enough for all the
partitions, created one slice and four partitions, and rsync'd it. But
how do I make it bootable?

Since I'm new to FreeBSD, it is with trepidation that I use fdisk,
boot0cfg, disklabel, etc. - I definitely don't want to endanger the
bootability of the main system.

Once I have some sort of backup, I want to take the next step: use a
USB backup disk so that it can be physically removed. That's what I
do on Linux: I use automount to sense the backup drive, the rsync to
it. I know FreeBSD has atd and so it has some similar capability,
right? But first I want to be able to boot off my PATA backup drive.

Thanks for any hints or advice!