I already discussed that a bit here:

https://news.ycombinator.com/item?id=26804794

So, some NAS vendors seem to spread a bit of FUD that their hardware is purportedly better or more robust than a self-built Linux system. It isn't - they are very likely using exactly the same chips as you. What you get when you buy a ready-made NAS is that it is quicker to initially set up and less work to configure, that is what you pay a good chunk of money for. On the other hand, you are not unlikely to later end up with a system without security fixes or upgrades, or which can't be moved to newer hardware, as was my experience. Also, the easy initial configuration is nice, but it won't help that much if you need troubleshooting or a more tailored set-up.

I had similar requirements as you, specifically:

- not too expensive

- plenty of space

- very energy-efficient, so I needed an ARM CPU

- fast Gigabit Ethernet

- needs Windows file sharing for my partners' Mac

- flexible enough to set up some home services

- needs to have updates and standard Linux software support in the long run.

- easy to back up

- some protection against sudden disk failure, using a RAID

Here is my solution:

- a Raspberry Pi 400 with keyboard: https://www.raspberrypi.org/products/raspberry-pi-400/ . It has a quad-core Cortex-A72 and four GB of RAM - it is likely much better than a low-cost NAS. Also, it runs from an SD card, which makes it much easier to install, update, or revert the system, you do not need to be afraid of bricking your device because that can't happen. It also comes with a nice and comprehensive printed manual if you buy the kit.

- Two external 2.5´´ HDD drives, 5 TB each, with USB3 interface (I took a Western Digital one and an Intenso one, as it is tradition to use different makes in a RAID, in order to avoid correlated failures)

- One powered USB 3 Hub with sufficient power for all of them. These can be relatively expensive, so leave room for it on your budget, you need it.

- an industrial-grade SD card for the Raspberry's system

- you also might want some kind of organizer to keep the devices together while providing for good passive cooling, like that: https://www.amazon.com/DecoBros-Cabinet-Basket-Organizer-Sil...

That might sum up to 250 - 300 €. If you want it cheaper, you could skip the RAID, allowing to omit the second HDD and the USB hub.

My set-up includes:

- standard Raspbian for the software

- I use the root-fs overlay provided here https://github.com/fitu996/overlayRoot.sh , in order to protect the SD card from constant writing

- /var (and with it the system logs) go to a different partition of the RAID

- RAID is set up with mdadm . There are good instructions on the web.

- I use ext3 or ext4 file systems for data. btrfs is in theory easier to extend but it has worse robustness characteristics than Ext4, and for a long time had no well-working filesystem repair tool if metadata gets corrupted. Some people might use ZFS but I think it is overkill and over-complicated for this case, it is not part of the standard kernel, and it might make kernel updates more difficult or impossible.

- also I set up openssh-server, Samba, Linux NFS server, a MoinMoin Wiki, rsync.

- I configured systemd so that Ctrl-Alt-Del on the keyboard will shut down the system, even if it is disconnected.

- I configured the disks to shut down when unused for some hours, using hdparm - that saves power, too.

The overlayfs needs to be activated when everything is finished. It has the effect that changes and logs are stored on the RAID. If you update the system, it needs to be deactivated.

Of course, back-up your SD card.

Also, please always back up your data - RAID is never a substitute for backups. What RAID is good for is that it increases availability of your device in case one of your disks fails. But there are many errors which it cannot help against, so you need regular full backups (like you need for any Synology or whatever NAS as well!!).