Error -50

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?
Mac OSX 10.4.2 a SATA drive on a Powermac G5 2.0ghz. It can't repair the drive or permissions. it says it can't unmount. it has worked before but this problem has occured too.
If the drives are internal, check Master/Slave settings on the back of your HD's? Make sure all the cables are plugged in tightly?
Have you tried booting from the Mac OS X Install CD and repairing the disk that way with the Disk Utility on the Installer CD?