+1 for a NAS, but you'll need to think the storage strategy through fully. Single external HDD is just another potential single point of failure; even worse if it's also the only place your photos are stored!
I use a two disk Synology NAS, a 214play, in RAID 1 configuration, which means each 3gb disk is an exact 'mirror' of the other resulting in a total of 3gb of storage.
Attached to the NAS is a usb 3gb external hard disk which is used to back-up the most critical files from the NAS.
(The 'play' versions of the Synology NAS give improved video decoding if you want to store videos for playback elsewhere)
On one of my PC's is a file containing all my photos, which is copied on a regular basis to the NAS, plus another usb external hard disk attached to the PC.
I also use Acronis back up software that copies an image of my entire PC hard disk to the NAS, plus another entire image to yet another usb external hard drive.
So, counting the two disks in the NAS, it's back-up, the PC and it's backup, plus the two Acronis images, I have seven copies of my photographs.
The risk, however, is that they are all in my house, so a fire would see them all destroyed.
I'm just in the process of looking for external storage, be it physical or virtual (cloud), that I can back up to too.
I might go down the route of installing another NAS at a family members house, as the NAS units can be accessed remotely.
Over the top? Yes, but it's a life time of photographic memories. The same equipment also gives me backups of all my other important files, etc.