Network-attached storage (NAS) is exactly as the name implies — storage connected to a network and available to clients on that network. A NAS enclosure is a small server made by brands such as Synology, Asustor, and TerraMaster, which only requires a storage drive (or a few) to get up and running. They also come with their operating systems and support but cost considerably more than building your own NAS from PC parts and installing a custom OS. Which is better depends on what you plan on using the NAS for.
DIY NAS vs. prebuilt server
Building a NAS is a lengthy process compared to buying a prebuilt enclosure and it's a similar story to a desktop PC. With a PC, you can purchase all the required parts yourself and put them together, but this requires knowledge and you will have to support the system without expert assistance. Another route is to go with a systems integrated (SI), which builds the PC, delivers it to your address, and offers after-sales support should anything go wrong. A NAS is almost identical with prebuilt servers available for those who'd prefer not to build one from scratch.
There's no right or wrong answer here and it all depends on your technical knowledge and how much time is available. When working with sensitive data, it may also be wise to have support channels in place to keep your infrastructure up and running. Any downtime could cost a business money or a household some frowns with the media server offline. A prebuilt enclosure will have less capable specifications than a custom PC build since NAS manufacturers use low-power chips and SODIMM memory slots.
They can also be closed systems, meaning you won't be able to load a different operating system on the NAS. TerraMaster is a brand that allows many of its servers to run an OS of your choosing, which is great for when you need something with more capabilities than the included software.
Why I built my own NAS
I've used countless NAS enclosures from many of the top brands in the space, but I've always wanted to run my infrastructure, especially for work. I have a media server running Plex and it's a synology enclosure with five drive bays and an Intel chip. These specifications are more than enough for what we use this NAS for, which is primarily streaming movies, music, and shows. I've traditionally used prebuilt enclosures for work as well as leisure and used custom-built NAS with PC parts for feature writing and testing.
That is until now. I have a TerraMaster T6-423 running TrueNAS SCALE, which was about as custom as I got with a live environment, but now I'm running the same OS on a desktop platform. Using the excellent Fractal Design Node 804 as the foundation, an AMD Ryzen 9 5950X processor is joined by 32GB of DDR4-3200 RAM, and six 4TB Seagate IronWolf hard drives. This provides enough capacity and expansion support for running a business and storing vital files for the operations of said business.
No official support is available from a company should I encounter any hardware (or software) related issues, but having built systems for the better part of 20 years, this is a non-issue. Knowing what's inside the system, having the option to swap parts out should something go wrong or I somehow require an upgrade, and saving money in the process by repurposing old hardware made it the right move with only the drives needing to be purchased. TrueNAS SCALE is also a perfect match for what I require from the NAS.
Build a NAS with an old PC
The easiest (and most affordable) way to build a custom NAS is by repurposing an old PC. This allows you to reuse existing parts and provide a new life for them outside of a landfill. Older PC hardware, even parts from multiple generations prior, will still be powerful enough to run a NAS server. Many prebuilt enclosures have low-power Intel and Realtek processors, limited system memory, and no discrete graphics, providing you with an edge with more capable components.