I really really liked the HP MediaSmart Windows Home Server that I bought years ago. It was one of the best Microsoft devices that I have ever purchased and just sat in the corner doing its job quietly. Since it was awesome, that means Microsoft created a new version that sucked and then discontinued it. In Microsoft's defense they have baked a lot of the functionality directly into Windows 10, but I wish they would have kept the Home Server platform alive. My Windows Home Server is still chugging along, but I am slowly migrating its tasks to a Synology RS816 NAS that I bought last year. I bought the RS816 specifically because it was the cheapest rack mountable unit that Synology makes. So far I am enjoying having it as an addition to my network and it has a host of features that just work, including storing photos, security camera monitoring, and VPN access.
The RS816 has built in slots for 4 drives and I started out with two 4TB Western Digital Red drives in a RAID 1 configuration, giving me a redundant 4TB of storage. I store movies on it in an MKV format and Blue-ray movies in particular can be over 20GB in size so I quickly started to fill up that 4TB of storage. Synology has made adding space a very easy task and I put together a video that walks through installing new drives, from physically putting the drives into the RS816 to configuring the drives in the DiskStation web interface. I added two more 4TB Western Digital Red drives giving me a total of 12TB (4 X 4TB drives in a Raid 5 configuration), using one of the drives for redundancy.
The one thing that stood out in the process is that it actually took 2 days for the RAID to be reorganized onto the new drives. The original 4TB was still available for read and write access during this time and after 2 days of the NAS working through things the additional spaces was finally available.