It can hold up to 6 internal harddrives:
https://www.cnet.com/products/ecs-geforce6100pm-m2-2-0-motherboard-micro-atx-socket-am2-geforce-6100/specs/
2 ATA and 4 SATA devices.
You can consider adding USB harddrives, or even better replacing your current harddrives by bigger ones.