Please elaborate greatly on your situation. "I can't write to my hard drive" is extremely difficult to even begin troubleshooting.
What kind of Macintosh are you using? What version of OS X are you using? What is the "second hd?" Internal? External? ATA? SATA? SCSI? FireWire? USB?
Have you checked the permissions on the drive? Have you ever been able to write to the drive in the past?