you could use SCSI drives and they work well for that, no MObo needed , just set the case next to the PC, you might have to "hot wire" the power supply and run the scsi cables over.
otherwise, setting up a data storage tank can be done with linux, not as easy if your unfamiliar(but widely documented), or any windows OS, standalone software is available if its not XP/2000.. get a big case and an extra PCI IDE/SATA controller (depending on your drives available) you can even get linux that runs on cd-rom/diskette(the diskette versions are not very easy to work on), this requires a little skill to get your options setup and burned to the CD/loaded off a drive. the benefit is the OS is almost impervious to damage once its setup, if something bad happens, reboot.
I've set some of these up before, How were you planning to access the data? network to more than one PC? or just 1 computer needing more data space?