Always make sure you back up your NAS to an external drive regularly. Most NAS's include the ability to backup to an external drive. You might think that using multiple drives in RAID config will protect you but I can tell you from my own painful experience that this isn't the case. Listen and learn.
My original Qnap NAS had 2 x 500GB drives in RAID1 (mirrored, redundant) and gave good service for many years until the power supply started to fail (presumably the caps started to dry out). The first I knew about it was when both drives became bricked. This shouldn't happen with a RAID system surely? One of them was completely trashed and the other was only accessible by removing it from the NAS, mounting it (EXT3 format) and manually copying each directory to an external drive. If I went into a corrupted directory, the PC would BSOD. I had to work this recovery process out myself, as the forums were of no help at all in this instance.
I only recovered about about 70% of the files added since the last full backup and lost quite a few hours in the process. Not long after replacing the PSU and both HDDs with proper "enterprise" grade drives, the fan failed (bearings had dried up) and the OS became corrupted, trashing the data again. It's now in the back yard waiting for a trip to the dump.
The HDDs now reside in an Asustor AS-202TE (2 drives in RAID1). I lost faith in Qnap personally. Although I'm sure they are fine nowadays, I decided to change to another brand.The latest NASs have masses of apps and extras built in, such as media servers, browsers, HDMI, multiple USBs, wifi, webcams etc. I plugged mine into the TV with an HDMI cable and now use it to watch BBC iPlayer, Youtube, browse photos and play music (no PC required). Not bad for under 200 quid. You can access it from anywhere with an internet connection using the apps that come with it and it even sets up port forwarding etc automatically. Mine is an Asustor but the offerings from Qnap, Synology etc offer very similar features. I always back up to an external drive every month or so and back up critical stuff to a cloud drive.
Murray