I am wondering if I can purchase a stand alone hard drive to store this media and some how access it still through Itunes and Apple TV?
Similarly this can be done with your iPhoto and iMovie Libraries as well.
Here are some ideas/suggestions for freeing up your storage space on your iMac's internal hard drive.
Hard drive getting full or near full?
Do a search for and downlaod and install OmniDisk Sweeper and OnyX.
Here are some of my tips for deleting or archiving data off of your internal hard
Have you emptied your iMac's Trash icon in the Dock?
If you use iPhoto, iPhoto has its own trash that needs to be emptied, also.
If you use Apple Mail app, Apple Mail also has its own trash area that needs to be emptied, too!
Other things you can do to gain space.
Delete any old or no longer needed emails and/or archive older emails you want to save to disc, Flash drive/s or to ext. hard drive.
Look through your Documents folder and delete any type of old useless type files like "Read Me" type files.
Again, archive to disc, Flash drive or ext. hard drive and/or delete any old documents you no longer use or immediately need.
Uninstall apps that you no longer use. If the app has a dedicated uninstaller, use it to completely uninstall the app. If the app has no uninstaller, then just drag it to the OS X Trash icon and empty the Trash.
Also, if you save old downloaded .dmg application installer files, you can either archive and delete these or just delete the ones you think you'll never install, again.
Download an app called OnyX for your version of OS X.
When you install and launch it, let it do its thing initially, then go to the cleaning and maintenance tabs and run all of the processes in the tabs. Let OnyX clean out all web browser cache files, web browser histories, system cache files, delete old error log files.
Typically, iTunes and iPhoto libraries are the biggest users of HD space.
If you have any other large folders of personal data or projects, these should be thinned out, moved, also, to the external hard drive and then either archived to disc, Flash drive or ext. hard drive and/or deleted off your internal hard drive.
You can purchase standalone external hard drives that can act as both a backup drive and media and data storage, too!
Also, if you want faster transfer rates and a faster, bootable OS X external drive, depending on your model iMac,
I would look at FireWire 800 drives instead of USB 2.0 drives.
You seemed concerned that even 3TB drives may not be large enough for you.
Check out Mercury Pro hard drives and OWC ( macsales.com), external drives from G-Technologies and from LaCie. For LaCie, their D2 drives come in up to 4 TB storage size config.. The D2 line come in FireWire connection protocol. Lacie's larger network Big Disc drives come in storage configurations from 2 TBs up to a whopping 20 TBs. These drives are more or less just for data storage and only use USB 2.0 for a connection protocol. You can still make these drives a bootable OS X drive, they will just run a little slower in comparison to a bootable FireWire 800 connected drive.