To specifically answer your question would require a bit more information than you give us, but perhaps this will help:
With a standard RAID controller (Intel, Adaptec, LSI, others...) or if the RAID volume is being created through the operating system, a section of a given drive can be allocated for inclusion in a RAID volume. While this is usually the entire available hard drive space, it does not have to be...
Check the RAID controller's BIOS/Firmware (accessible at boot time: keystroke combination varies by manufacturer) and verify that both drives are being seen (you state that there are two 500GB drives). While by no means an accepted or acceptable practice, it is possible to partition a drive and designate both partitions as members of the same storage array (I've done it....), so it is possible - unlikely, but possible - that this is the case. More likely, you have both drives set with 250GB partitions and the second partition(s) on each drive either inactive or hidden.
If the array was configured via the OS, check the disk management control panel (in Windows) or the equivalent conf file in Linux to see if the drives are being recognized properly and used in their entirety.
I don't know if you built this system yourself, but it almost sounds as if you have a two drive, 500GB system (two drives totaling 500GB, or two 250GB drives).