Where were your writing them from and to?
Normally you would do a Copy operation, which would leave the original files in place on the source media. NEVER do a Move operation from one drive to another. Always Copy, then verify, then delete the source after verifying.
The important thing if you are going to recover files is to stop using the machine. Every minute you continue using the machine increases the chance that the spaces on the disk where the deleted files are will be overwritten with other data.
You can try various software to see if the files can be recovered
DataRescue http://www.prosofteng.com/
FileSalvage (Mac) http://subrosasoft.com/OSXSoftware/index.php?main_page=product_info&products_id=1
ActiveUndelete http://www.active-undelete.com/
PCTools FileRecover http://www.pctools.com/file-recover
Recuva: http://www.recuva.com/
There are some websites that have tools you can try, and then if it looks like it can recover some data, you have to pay to do the recovery
VirtualLab http://www.binarybiz.com/
If all else fails, you can send your drive to a data recovery service, it will cost hundreds or thousands of dollars depending on what the problem is.
If you are going to send it out for recovery, the less fooling around with software you do with the drive on your own, the better.
Seagate Data Recovery http://services.seagate.com/consumer_solutions.aspx
DriveSavers http://www.drivesavers.com/
ActionFront http://www.actionfront.com/
TotalRecall http://www.totalrecall.com/