Many use a NAS for backups but a NAS unit can also be used for general storage. Many have 2 disk NAS units but these tend to be less reliable compared to 4 disk models. The larger 8 disk NAS units are more tolerant of disk errors.
Some shops even have more than one NAS unit which is a growing trend. NAS targeted hard disks are usually limited to 8 disk capacity units. For those who need more capacity a second NAS can offer more storage. Moving data over the LAN tends to be slow with Ethernet. Typical throughput is 110 MB/s.
The larger NAS units may have an option for 10GBASE-T which is dramatically faster. Network switches are now much power in cost making this an option for the server rooms. High density 10GBASE-T is not that expensive but the performance is excellent. Upgrading the network has the best price performance for 2022.
Most 10GBASE-T switches have 40 gigabit backplane ports for expanding the network to have thousands of servers. 40 gigabit is being phased out in favor of faster 100 gigabit speeds.
Hard disk capacity has been sluggish to adopt larger capacity points over the last few years. The 20TB disks are the current capacity but availability is limited as corporate demand is high. The average selling prices for server grade disks has eased making them viable for NAS use.
Shingled drive managed hard disks perform poorly with NAS units. For that reason there was a real flap over the changes. The Seagate EXOS and Toshiba MG disks are designed for heavy NAS workloads. The WD Red Plus are suitable for moderate workloads such as a media server or backups.
Upgrading the disks in a NAS can be done as per the manual. The easiest way is to remove all the data elsewhere and then replace all the disks at once.