Probably have the jumpers in the wrong configuration--when I was making my new build, it took me FOREVER to get my SATA and IDE drives showing up at all in the bios and in the proper master/slave configuration (I'm pretty sure one of the drives had an incorrect jumper setting table).
Have you checked the manual for the two drives to make sure you have one set as master and the other as slave?
With all due respect, Hastibe, master and slave apply to IDE drives. Not SATA.
My girls (that's wife and daughter) have a different Dell and I found that the BIOS does indeed not see a newly attached SATA or ESATA drive. In this one, each SATA port is individually enabled or disabled in the BIOS. A pain in the rectum with my ESATA backup drive. Enable the port, attach the drive, backup, remove the drive, disable the port. If you leave a port enabled with no drive attached, it complains / pauses during the boot.
So go into BIOS and look for the various SATA ports. See if all but the system drive are disabled, guess which one you attached the new drive to and enable it.