can i mirror the hd on one machine to a hd on another machine or should i setup similar systems on each and then mirror a particular partition(which will be a backup in essence)?

I have used Power Quest Drive Image to take images of linux disks and successfully used it to install on another machine. The only requirement is that the drive must have the same id. i.e. if the drive id is /dev/hda1 then the destination must be the same else you'll have boot problems and you'll have to edit fstab etc to get it to work.

There is a good linux tool for the partition images: partimage
It worked flawlessly for me every time I used it (and I did this quite often when exchanging harddrives - even a Windows98 copy to another hd worked).

AFAIK it is included in knopppix so that would probably the most comfortable way to use it

But - like TigerOC said - take care of the fstab and probably you will have to set up lilo again. And if the hardware is different (graphics card, nic, soundcard etc.) you will have to take care of that too.

thanks to you guys for getting on this. partimage was mentioned in another thread and i went to the site(which i thought was very good) and got the download, although i also have knoppix(i'm impressed and i'm a slacker) which i downloaded and burned not too long ago(so it probably is on it too). i was asked if a debian system on a laptop could be mirrored to a pc, but i think what is wanted is a debian system on the pc which has the same apps available with data mirrored to start and then frequently synched to the laptop. so, i may be looking into how to readily update the pc data from the laptop.
powerquest seems to be for windows from the page i browsed, though i didn't go past that finding and i may have missed something(obviously, i did from TigerOC's reply), so i'll look again.
i do appreciate your time and thoughts on this, and i'm probably not done with it yet, but it's 5:30am here and i've got to crash or i won't get up before 2 or 3 in the afternoon.

When using drive imaging the os is not operational. Drive image uses boot disks and works in DOS and Partimage also uses a boot disk. I made a cd boot disk for partimage but have not really got to grips with is yet as I have used Drive image for a number of years for doing backups of Windoze systems.

Never tried it, but would

dd if=/dev/hda of=/dev/hdb

work? The first harddrive you want to mirror would be hda, and the harddrive to be mirrored to would be hdb.

i thought about dd, and aside from being a tyro in its use, it seems to me that there might be problems using it to mirror a partition because of the way it handles file sizes-not that you would lose data, but things wouldn't be "mirrored" in an absolute sense. this may be handled with options, but as i said, i'm no expert and would need some experience.
your suggestion, KneeLess, has got me thinking to look further, and i thank you.