With a NAS device, you may also need to fiddle around a bit more with the physical installation. While vendors such as Seagate and WD supply NAS devices that come in pre-configured capacities, the usual form for other vendors, such as Asustor, Qnap, Synology, and Thecus, is that you buy only the case, which is empty, and then install the hard drives of your choice yourself.
Your buying decision in such a scenario will have to include how many drive bays you want. The more drive bays (up to four is most common for consumer NAS devices, though six and eight are available, too), the higher the capacity you can install. Two drive bays should be the minimum that you go for, though drives with one bay also exist if you want the simplest installation experience.
If you are going for an empty NAS, then you will need to buy 3.5in hard drives of the capacity of your choice and follow the recommended NAS vendor's instructions on how to properly install them physically, and then how to set them up after that. You can install only one drive (even in a multi-bay drive), but it is recommended that you install two or more in order to keep your data safe.
Since most NAS devices have two or more bays, it is typical for a NAS device to support RAID arrays, with RAID 1 being the recommend array for a NAS device with two drive bays. What RAID 1 does is mirror your data on both drives. If one drive dies, then your data is not lost and you can replace that dead drive and rebuild the array to keep protecting your data. If you were using RAID 0, which stripes your data across both drives for the purpose of speed, then a faulty drive would leave your data lost forever.
Note that with a RAID 1 array, your total capacity will be halved. If you install two 4TB hard drives, instead of getting a full 8TB, you will get 4TB as one drive will be used for data duplication. It is the price to pay for protecting your data. It used to be said that you should match the make and model of drives in your NAS device. These days it is common to mix and match vendors and models to minimise the chances of buying two drives that can develop the same fault. There are also dedicated NAS hard drives these days, from both Seagate and WD, which are optimised for use in a NAS device.
You should buy matching disk capacities in order to make the most of the total capacity. For example, if you buy one 4TB drive and one 2TB drive, a RAID 1 array will only let you use 2TB, thereby making that extra 2TB go to waste. Buy the same capacity for both drives, and that maximum will be used in RAID 0. Mixing capacities works in a mode called JBOD (just a bunch of disks), but you don't get data redundancy.
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