This error looks like an issue with the Pi connecting to your external drive. If your external drive only has a USB connection and no direct power I’d expect you to continue to have issues.
As an example of what to look for, I found the ORICO USB 3.0 External Hard Drive Enclosure on Amazon. I’m not recommending this model - I haven’t used it. BUT if you look at this enclosure you’ll see it has a power supply to power the drive and then a USB connection to connect to your computer (Pi). This is what you most likely need to get your drive working reliably - a drive enclosure with it’s own power - the Pi can’t supply enough power over the USB cable to be reliable.
Alternatively, you can forego the external hard drive and just use a larger microSD card in your Pi and then run everything off the microSD. I’m running a Pi right now w/a 1.5TB microSD card.
Reading through this thread, I also saw you asked for device recommendations (rather than a Pi). Start9 offers preloaded servers at https://store.start9.com/ but you can certainly source DIY hardware yourself. There is a forum post with various hardware options that people tried and got working at: [Known-Good Hardware Master List] Hardware Capable of Running StartOS - #155 by Comza