You can attach external harddrive.

You can get those Raspberry pi like products and attach internal ssds.

You can get those tiny almost android box sized PC's and have internal ssd, harddrive.

Or simply buy android box with 128 gigs of storage.

I don't know of any that hold a hard drive. At least not a reputable one I'd buy. You can get the new Chromecast and a compatible USB hub. External storage can be added along ethernet if desired. A more DIY option is Raspberry Pi. There may be a way to convert an SATA connection to USB, but that's not in my wheelhouse.

Whatever you buy, make sure it's reputable and officially supported. Boxes that aren't supported or properly licensed -- whether they're running Android or Android TV -- often won't play some content, like in HD. That's especially true for Netflix. And cheap boxes from China rarely get any security updates.

but I'd love for the box to play media files. I was hoping to find one that could hold a regular sized hard drive as I have a few spare.

You might be interested in this device then.

Run CoreELEC Kodi on it and it will literally play anything you throw at it:

https://discourse.coreelec.org/t/s922x-h-bee-link-gs-king-x/11634/2

Downside Bee-link GS-King X is it's expensive a F for what it is.

Which is why you are better of buying a cheap AMLogic S905X3 device, like a $65 Ugoos X3 Cube and run CoreELEC Kodi on it and add an external USB3 hard drive.

CoreELEC Kodi has an inbuilt SAMBA file server so you can transfer files over a home network to attached hard drives:

This just released ODROID HC-4 - 4K HDR + HD audio - NAS solution would be very good for local media playback using the CoreELEC (Kodi) OS:

https://forum.kodi.tv/showthread.php?tid=357831&pid=2983276#pid2983276