Using the mi box as a NAS would be a bottleneck as it only has usb2 on it. I'd recommend checking if your router supports Samba via usb. Samba will allow you to login to view your files on almost any device running on the network and is baked right into Kodi itself. Another option, which may require you to be a bit more tech savvy is to build a NAS from a single board computer such as an odroid or the Rock64. Both can run Samba/ftp with great networking capabilities plus they're capable of running Sonarr, Radarr, and many other softwares that automatically add media to your library.

I am sorry I don't have any knowledge on this. But I am answering to let you know how I do it with other devices.

But I share my HDD from windows 10 pc using SMB (you can Google how to setup SMB ). Over the network to access it on mi box.

Also I have a USB stick attached to my Router which has SMB in built into it. So if you have a router I would suggest using that.

Samba server by ice cold apps should do the trick - I've not tried it myself however.

Linkme:Samba Server

Install AndSMB on the mibox to network mount the external drive. On your windows pc you can now see the mibox attached usb drive on your windows network list. Transfer speed will be slow due to many bottlenecks. The best way is to build a small odroid nas. That way you can buy a .com of your own to assign to the nas and go to it like you would Netflix or Spotify but with your own content. Check out "emby" once you finish with the first task.

Does your router support external drives? That would be way easier to plug it in directly to the router and then every device can access it.

I'll let the Android experts recommend an app though, seems inefficient to do it that way.

People are going to tell you what apps to do this with.

I'm going to tell you to not do this. It is going to be painfully piss slow.

Plug the Hard Drive into a modern home router that had inbuilt SAMBA (SMB) file sharing would be MUCH easier.

The easiest Plug N Play device for this type of file sharing is the Gigabit Ethernet equipped - LibreELEC Kodi boxes. That have a built in SAMBA file server included as part of the Operating System.

https://libreelec.wiki/accessing_libreelec#tab__sambasmb

Thanks everyone for your informative comments, many options for me to mess around with and try out going forward. Appreciated. Cheers.

Anyway you can do it the other way around? Connect drive to PC or Router or Server and then access it over the web from a file manager app on MiBox

One other possibility is to sideload the Teamviewer Host (not regular Teamviewer) app on your Mi Box and connect to the storage from your Win10 PC using the Teamviewer desktop app. I attempted to use this for remote control of my Mi Box which as it turns out isn't supported (only remote viewing)...however an added side benefit is that you can manage the apps and access the storage on the Mi Box remotely, so you could use that to access the mounted USB drive.

Link: Teamviewer Host APK

I have the opposite problem. I can't see my Nas through Kodi installed on mibox3

At my sisters house, I use a Raspberry Pi 1 Model B as a NAS. With OpenELEC, its basically Kodi and Kodi will let you share out any content connected to it. Other Kodi clients will have no problem seeing it as a UPnP device. I have this setup at my sisters house, she can access content from 2 different media players. On her tablet, she uses VLC.

NOTE: The latest Raspberry Pi3 Model B+ has faster Ethernet, should push 300mbps and work a tad better than the previous models. If you use a MiBox, it might be slower as its still limited to USB 2.0 speeds. Im not sure what the Pi3 B+ is doing, but its still using USB 2.0, but is able to get faster speeds.

From Mi Box play store, download X-Plore, it's a native File Manager for Android TV. When launched, find WiFi-Server in it's menu, enable that and note the IP:port. Put that on your PC internet browser of choice. You can now view, edit, upload files on your MiBox from your PC.