WARNING: Once I put OpenWrt on my FileHub Plus, I couldn't restore back to the original firmware. I am not saying it is impossible, I just couldn't find any working directions.
Using the directions in the OpenWrt Wiki ( https://openwrt.org/toh/ravpower/rp-wd03 ) I was able to get OpenWrt installed. I ended up using a snapshot build, which didn't have LuCI installed so I had to figure that part out. As a note, the directions under the "Installation section" shows two files being renamed to kernel and rootfs, but when you look lower in the "OEM installation using the TFTP method" section you will see the file two method is used for snapshot builds and recent releases. As of right now the latest stable build is 19.07.8, you can grab the correct file using the firmware selector ( https://firmware-selector.openwrt.org/ ). You end up renaming that one file to "kernel" when you share it via TFTP, instead of the two file method for snapshot builds.
While getting familiar with OpenWrt, I started to play around with adding software and quickly ran into space issues since the WD03 only has 8MB of storage. At that point, I set the WD03 aside and shifted my attention to getting OpenWrt working on my Raspberry Pi 4. I might pull together another post later on that.
Once I was a bit more comfortable with OpenWrt, I decided that I needed to do something with the WD03. I tried my best to restore the original factory image, but despite following the directions I could find online nothing worked. For some reason I had it in my mind that the low RAM was the issue, but then I remembered it was the storage space not the RAM. Next thing I did was start to read up on what I could safely remove from OpenWrt. It turns out that, due to the way OpenWrt uses SqashFS you don't really save space just by deleting files. The best option to save space on OpenWrt is to do a custom image.
So making a custom image was my next focus, as always first step was to read up on it a bit. The OpenWrt Wiki ( https://openwrt.org/docs/guide-user/additional-software/imagebuilder ) again was very helpful. I used those directions, and the information on saving space ( https://openwrt.org/docs/guide-user/additional-software/saving_space ) to setup a custom build for my WD03. It worked great, and best part is while I am messing around with the WD03 if I decide to reset things my custom build already has LuCI installed so I am ready to go right from the start. I even set it so the OpenWrt wi-fi access point was on by default so I could easily connect and change settings. I would probably avoid putting anything too specific in your custom build, for example don't save your home wi-fi password since one day you might get rid of your device and you don't want certain information saved on the device even after a reset.
Overall, if you are looking to play around with OpenWrt then go for it - just make sure you read directions carefully, and understand you could mess up your device if you are not careful. I don't think I can get my WD03 back to stock firmware, but I am ok with that. Of course, having an understanding of things like networking and linux commands is very helpful, but I am guessing the type of person who is interested in OpenWrt is probably familiar enough with those concepts or willing to learn.
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