Hello,
What kind of disk configuration are you running at the moment?
If your configuration does not allow you to expand an existing array then adding another drive (and creating a new volume) you could configure so that for example your logfiles, backups etc. will be stored onto that one.
If you are running SQL2000 you can also reconfigure that for exmaple the SQL Transaction logfiles will be stored on this other disk (this however takes some configuration within SQL server).
In one of our environments I am running HW mirrored disk on the webserver having two partitions. One for the system itself and the second partition for all the logfiles and IIS data, backupfiles etc. Then I have a separate machine for the SQL databases, which is configured with a RAID5 which allows me to simply add more disks and expand the RAID array
Regards,
Thomas