I wanted to do this since I saw the specs of the pi4, but only had time now. It's running as a NAS in my local network using samba. There are two WD 3TB HDDs connected that run as raid 1. Unfortunately, I can not connect both of the directly to the pi, there still seems to be not enough power and I sometimes get a power warning (red LED blinking). So I decided to get a Usb3 hub with external power supply (Icy Box AC611). It comes with a charging port and my plan was to connect the pi4 there for power, the two HDDs into the Usb ports and in this way would only require one power cable. But I ran into the same problem with a sometimes blinking red LED, so in the end the pi got its own power supply. Everything is stacked into one of these four layer racks that are made for raspberry pis. The pi on top is screwed onto the rack, the HDDs are big enough that they don't move around and the Usb3 hub at the bottom is fixed with Velcro. I'm also running nextcloud on the pi which provides an easy to use interface that I can access from outside and allows sharing of files that I have stored on my NAS. The move from my old pi3 b+ to the 4 was quite simple. I started with a fresh buster install, added all the packages that I needed and copied the settings from my old pi to the new one using rsync. The same worked for the nextcloud install, where I just rsynced the files in www-data as well as everything belonging to mariaDB. From starting to have it running again it took about 3h in total. As others have already reported, I get read and write speeds of 40-50 MB / sec. I might be limited by the speed of the HDDs at this point and might upgrade to SSDs in the future. However, WLAN caps at around 10 MB / sec.
