What's inside a NAS
A NAS is a small computer built to serve files. It has a low power CPU, some memory, one or more drive bays, a network port and an operating system that handles shared folders, users and RAID. It shows up on the network as a file server. Windows PCs usually connect over SMB, Linux and Unix systems over NFS, and most NAS units speak several protocols at once. A USB drive hangs off one computer. A NAS hangs off the network, so everyone sees the same files.
Take the Synology DS923+ as a concrete case. Its spec sheet lists four drive bays, two M.2 NVMe slots, two 1GbE ports and support for RAID 0, 1, 5, 6 and 10 plus Synology's own Hybrid RAID.
Picking drives and a RAID level
The number of bays sets your options. Two bays mean RAID 1 or RAID 0, and RAID 0 is the wrong choice for a file server. Four bays open RAID 5, RAID 6 and RAID 10. With 8 TB drives, a four bay NAS gives 24 TB in RAID 5, 16 TB in RAID 6 or 16 TB in RAID 10, and a two bay NAS gives 8 TB in RAID 1. The RAID Calculator shows each layout for your own drives.
Buy drives made for the job. NAS drives are rated for 24/7 use in a bay next to other spinning drives, and their firmware is tuned for RAID. Seagate lists Error Recovery Control on its IronWolf drives, and WD calls its version NASware. Both brands also publish a workload rating. WD rates its Red Plus drives for 180 TB a year and its Red Pro drives for 550 TB.
The network is the real speed limit
A gigabit port moves at most 125 MB/s, and real transfers land a bit under that. One hard drive reads 160 MB/s in the model the Throughput Visualizer uses, so a single gigabit port holds back even one drive. Copying a 4 TB library to the NAS takes about 8.9 hours at that speed. A 2.5 GbE port carries 312.5 MB/s and a 10 GbE port 1,250 MB/s. A 10 TB backup takes 22.2 hours over gigabit and 2.2 hours over 10 GbE.
What a NAS doesn't do
RAID keeps the NAS running when a drive dies. It doesn't back anything up. A deleted folder is deleted on every drive, ransomware that reaches the share encrypts it in place, and a fire or a power surge takes the whole box. Keep another copy on a different device and one more off site.
A NAS also runs all day. Synology lists the DS923+ at 35.51 W in access mode, which comes to about 311 kWh over a full year. With the drives in hibernation, the sheet's 11.52 W works out to about 101 kWh. The 4 bay NAS power cost page prices that at your own electricity rate.