Network Attached Storage (NAS) devices provide reliable data storage for many devices. They are necessary in BYOD enterprises. In order of desirability (worst 5o to best 1) the options include:P
not recommended: proprietary office NAS, external drives on desktops/devices
7. A common approach is a "office" NAS like QNAP. These use hardware or proprietary firmware RAID storage.
Advantages: Off the shelf, "supported". Many different models that appear to provide one that will serve you well.
Disadvantages. The "support" doesn't extend to your data. Sophisticated configurations (RAID5, RAID6) have serious problems when a board fails, as the firmware and hardware change often and are hard to reproduce. Companies are small and there is a risk they'll just disappear. If they do, get your data somewhere else fast.
These products rely on hardware RAID, which is problematic as it's impossible to reproduce and restore configurations exactly between versions of hardware and firmware and other software. Responsible options (2, 1) specifically advise not to use hardware RAID.
6. External hard drives hooked up to routers or media devices or desktops (or worse, laptops) shared by them across the network.
Advantages: faster than an "office" NAS sometimes, as a desktop PC has more processor and RAM available.
Disadvantages: far less reliable as it depends on the machine being physically on and often at the same static IP address. Loads the desktop machine and its net connection to do two jobs, which may not matter if it's dual gigabit Ethernet or handling few larger files.
4. An SSD only / M.2 NAS
These are proprietary too, e.g. Qnap TBS-453A Amazon, review from benchmarkreviews.com, review from GeekLingo.net. This device still has sub-gigabit transfer speeds despite having an integrated 4x1GbE switch, but hooking up teamed gigabit Ethernet can achieve close to this.
For storage this uses 4x M.2 ports in a small form factor with no spinning HDD. It has an SD card reader & can be expanded to up to 8GB ram. It has HDMI out & due to silence & no spinning metal can be ideal to carry to 4K presentations.
open but dedicated NAS options
All of the following require a true gigabit home network, so expect to require an 8-port gigabit switch. Off-loading PoE devices (VoIP wired phones, security cameras, WiFi access point onto their own 4-port PoE switch is advisable to save gigabit ports & power and make it easier to ensure they run in an outage).
cheap home NAS from very large disk vendor
3. A Seagate Central - very slow storage (0.1GbE) & a very bad proprietary Seagate OS. However it uses Linux data drive formatting, so it's possible to recover data by plugging it into another Seagate (there are many types all identical at the drive interface) or a real Linux PC.
3a. Other home NAS from a huge OEM like WD or Apple has a very good chance of being available used at some price, has only a few well documented firmware versions you can always find on their website… and only handles one drive not a RAID configuration that may not be readable in the next version of the firmware.
Advantages: cheap, fast enough for any image / photo and book / PDF and compact encoded 720p (H.264) and very compact 1080p (H.265/HVEC) video files. Reliability issues can be dealt with by simply buying more devices and redundantly backing up on every one you have. When a disk or a board goes, you have spare parts, assuming you bought 4 or 5 of the devices at once. Put up a 'wanted' on kijiji for them, also, pick them up from people stupidly upgrading to QNAP etc.
Disadvantages: no size > 4TB, if you want redundancy you need another NAS of the same size to mirror it manually. Simply not fast enough for 2160p/4K video or larger even with excellent compression, forcing a cacheing approach.
Worst disadvantage: Very slow processor, flaky OS that sometimes renames folders, and only one gigabit Ethernet port. Expect delays. For Mac users, expect to require a better Samba solution.
true "enterprise class" NAS
2. A proper enterprise class FreeNAS/BSD/Solaris continuously available file server. The lowest end of which are FreeNAS certified. These must be configured with ECC RAM and reliable UPS, so it can run RAID or ZFS safely. ZFS or acceelerated ext4 being the only filesystem that can actually saturate a gigabit LAN and make use of dual gigabit or 10 gigabit connection.
Expect to spend a grand on this, but then expect it to easy to recover your RAID without dealing with proprietary firmware incompatibilities cough QNAS.
Advantages: No kiddie stuff. This is what pros use. This is all pros use. You can go into any data centre in the world and find this configuration, and probably find a job doing it as well. Keep in mind that autonomous data sites (top secret, on ships, etc.) need this kind of configuration for going offline.
Disadvantages: Cost. Learning curve possibly not worthwhile for those who intend to spend their whole lives „in the cloud“ letting someone else deal with all this and never expect to go offline or deal with a power outage.
- Samsung 960 PRO NVMe M.2 SSD or better, i.e. the motherboard must have at least two preferably three M.2 slots.
- One to four Enterprise-class 64-bit multicore processors, e.g. XEON, Gigabyte sells some ECC boards in this class for under $400 as of 2016-04
- 32GB ECC RAM Minimum (1GB per TB of storage is a good rule of thumb but might need to be adjusted depending on workload/application) - this requires 2x 16GB DDR3 or 4x 8GB DDR3
- Two mirrored 16 GB Boot Drives (USB or SATA DOM recommended) - speed not critical
- High-endurance Flash/SSD-based write log device (ZIL) for synchronous writes only (Flash/SSDs with “Power Loss Protection” recommended). Two devices mirrored if uninterrupted performance is critical.
- High-performance SSDs for read acceleration if the most-requested data doesn’t fit in RAM and the random read load is high.
- Data storage supported by FreeNAS
- At least 4 direct attached disks (Hardware RAID strongly discouraged. It reduces the data protection and recovery features of FreeNAS considerably.) 
- If necessary to add disks above what the motherboard supports, do not use RAID cards. Host Bus Adapters (HBAs) are recommended instead to give FreeNAS/ZFS direct access to the individual drives. LSI HBAs are the best choice with FreeNAS.
- At least two physical network ports (dual gigabit Ethernet), ideally 4. Intel recommended for 1GbE. Chelsio recommended for 10GbE. Gigabyte boards have 4+1. nothing not on the FreeNAS support list
2 or 4 Ethernet port FreeNAS or Linux dedicated file server
1. A slightly lower end FreeNAS or prosumer type Linux file and local web server, probably an old desktop or a new miniITX box capable of holding 3t least two 3.5″ drives at least 3TB each, in either ext3 or ext4 or a RAID1 (or if you insist RAID0+1) configuration. If you’re smart you’ll use a server-class miniITX board with FOUR GIGABIT LAN PORTS on it, not some toy HTPC board. And you’ll have 4 RAM slots and will stick to a lower power processor so your power backup lasts.
You could run FreeNAS on this, but if you use say Ubuntu Studio already then using what you already have to support may be wiser. Judgement call. The FreeNAS requirements are:
- Multicore 64-bit* processor (Intel strongly recommended) - no 32 bit hardware, though FreeNAS 184.108.40.206 still supprots it, and UFS filesystem, which requires only 4GB boot and 4GB RAM.
- 8GB* Boot Drive (USB Flash Drive suffices)
- 8GB* RAM
- At least 1 direct attached disk (Hardware RAID strongly discouraged) using motherboard or supported controller
- One physical network port
But you probably should double all those specifications. The off the shelf iXsystems FreeNAS Mini Storage Device has
- 16GB Boot Drive (USB Flash Drive suffices)
- 16GB ECC
- At least 2 direct attached disks (Hardware RAID strongly discouraged)
To reproduce this using a flexible carry-handled miniITX case that can handle two 3.5" drives like the Corsair 380T would cost somewhat less. The main expense is a 4x GbE mini-ITX mainboard with 4x ECC DIMM slots which should be populated with 2x 16GB ECC so they can be upgraded. Intel® Atom™ C2750 processor is very weak compared to (2. TrueNAS) but 1 x PCIe x16 slot for additional storage or network features, and separate management LAN port to access the board's remote management functions, are effective, since the PCIe x16 slot can accomodate a PCIe SSD for boot and write log and read acceleration (is it reasonable to do all these things on one device? even with very fast parallel r/w?).
There are similar options with faster XEON D-1541 processor or D-1521 but only 2 gigabit ports, and a higher end board with D-1521, 2 gigabit ports, 2 10-gigabit ports (Cortina CS4227, not the Chelsio recommended by FreeNAS). Unfortunately it's not yet possible to build based on server boards at pcpartpicker.com  but hopefully as server boards come into more use, this will change.
Advantages: Off the shelf everything, you can build this with parts from NewEgg (though you can also build an enterprise class NAS that way, expect some out of stock). If you get a carry handle case, you can haul this thing around and use it in a boardroom for a superfast demo, or for a zombie party, or to handle super secret onsite data, or something. Very flexible. When it’s obsolete you can shift out the storage, shift in a video card, and use it for Internet gaming (not CPU intensive, not graphic intensive) but only if it’s lower power.
Disadvantages: May not be as cost-effective as cloud backup or the home NAS if you are generating less than about a gigabyte a month of real user-created data you need to back up and keep available 99.9% of the time. Very weak processor compared to a true NAS. Data storage likely restricted with miniITX board.