Actually you have multiple options depending on what you want to do. I'm assuming the phone actually supports OTG (on the go) transfers etc which it looks like a samsung 7 does.
You can get a OTG cable adapter which is basically a micro usb to usb socket adapter and then use any normal usb hard drive. This is also the 'cheapest' one to replace if you ever change to a phone with usb c for example.
Depending on the hard drive you can get a replacement cable that ends has a micro usb rather than full size usb plug (some do come with micro usb) at the end. This is the second cheapest because it's just a cable being replaced.
You can get one of the more specialised usb sticks like this sandisk that has both usb and micro usb connections (surprisingly useful). You'll need to replace it completely if you change to usb c etc.
There's also the option of a wirless hard drive like this seagate and apps like airdroid (I use this but only on internal wifi so no fees) which basically allows you to access your android phone via a web interface on your pc.
Obviously there's the idea of a nas/usb stick in your router on your home network with remote access via ftp/nas software etc but this might be a bit techy