XP hard drive recognition problem

I think I must have an XP problem I have an 500g H/D Hitachi Deskstar in a system I am trying to rebuild after a motherboard problem, I have a 250 with the operating system and some software on and that is OK.
IT is an ASUS M2N32-SLI motherboard and both the 250 and 500 drives are SATA drives the 500 will show on the motherboard and also in device manager as being OK.
Go to my computer and only the 250 is shown try saving from a program and only the 250 is viable.
Has any body got an answer as to how I can put this right.

Did you initialize (format) the 500GB under Disk Management?

If not you need to.

Thank you for that, it worked well it was all that was needed,
The more I learn the less I relies I understand.

No problem.

Let us know if we can help further in the future.