There's a number of possiblities to solve your problems. I'll list a
few for you, so you can select what works best with your priorities:
1)Enough money to go for the optimum solution
1a)Buy a new raid tower and fill it with as many HDs as you want.
1b)Attach it to a second computer (maybe a replacement for your old
system) and hook it up to your local net. Configure the raid and make
a new filesystem.
1c)use rsync to make a copy of the old /var/mail (or for that matter
of the whole system) while the system is still running.
1d)once done, shut down the old system and hook up the new raid and
the old raid on the same system (the one that is going to be the
system which is going to serve the mail)
1e)rsync both arrays again to bring them up to date on the last
1f)mount the new raid on /var/mail and go multiuser
2)little money (your current hardware is not the newest, but it is
apparently enough for your current needs, so reusing parts that
proofed reliable might be a preferable option.)
2a)get a a harddisk with >27GB storage space (possibly a cheap
2b)shut down your system
2c)put the harddisk into the system
2d)start the system, in multiuser mode.
2e)format the new disk and rsync the contents of your raid on that
2f)go to singleuser and kill your delivery system. rsync again.
2g)unmount the raid and mount the single disk as /var/mail
2h)reconfigure your old raid (eg. add the 3 additional HDs to your
empty bays or change the existing HDs to bigger ones).
2i)mount the raid again and rsync the HD to the raid.
2j)go to singleuser and kill your delivery system. rsync again.
2k)unmount the HD and mount the raid on /var/mail
2l)go multiuser again.
3)even less money (if you use a separate controller for each HD you
should get a fairly fast software raid)
3a)get a few cheap IDE HDs (the same production lot would be best)
3b)shut down your system and put the HDs in. Reboot and go multiuser.
3c)Format them and bind them into a software raid (if you are
interested in this, ask for clarification, please)
3d)rsync from the raid to the software raid
3e)go to singleuser and kill your delivery system. rsync again.
3f)umount the raid and mount the software raid on /var/mail
There are a number of other options (and modifications to the above
options) which I will go into if you dont like any of the above.
However, I need to know what file system is on the raid. And what is
the exact name of the raid controller? Would you consider changing
Some of the newer file systems also allow you to grow them, meaning
that you can actually add logical volumes to them.
If you need help, feel free to ask for clarification on the option you
decide to take. If nothing unforeseen happens, your downtime shouldnt
be more than a few minutes. Preparation time should be a few hours.