The only way as far as I know is for you to acquire an IDE RAID disk controller which will enable you to combine the drives so that they appear as one. However this is probably not going to be economically viable bearing in mind that you can pick up new 30GB drives for around £100. I don't think that a software based solution exists.
(NB I'm assuming that you have 'standard' IDE drives and not the more expensive SCSI types in which case you would need a SCSI RAID controller)
[ 08 September 2001: Message edited by: Seat 32F ]