interanl harddrive problem macbook...

Hi I relatively new to computers. I changed my internal hard drive from a 120GB to a 320GB but didn't swap the data over before the change. I changed them back again but my 120GB isn't being read by my macbook, it'll only boot as an external drive...

any ideas on what to do?

Did you install OS X and all your applications on the new 320 GB drive? If so,
put the 320 GB back in your MacBook - and place the old 120 GB in an external USB hard drive carrier. Transfer your data to the new drive that way.

When you bought the 320 GB drive, you should have used Disk Utility to format the drive and then either SuperDuper or Carbon Copy Cloner software to clone the contents of your 120 GB drive to the new 320 GB drive.

Regards.