If the external hard drive is formatted as FAT32, your iMac will probably be able to read it and write to it.

Otherwise, I'm not sure when it was Apple started shipping SAMBA with OS X, but if you go to the System Preferences app, and look at the sharing settings, if there's a "Windows File Sharing" option just start that service and you're good to go. Just create a simple network between your iMac and laptop, and transfer the files using Windows file sharing.

Now, actually using the files on the laptop is a whole different issue, which you're on your own for.