On a box without rooting, one way is to use FTP server in MiXplorer. Because opening any port below 1000 on Android requires rooting, you want to set FTP port number to something like 2121. You can then use a FTP client, like WinSCP, to upload files from PC to your box. If your SSD is attached to Mi Box as "Removable Storage", it's read-only unless you use a custom USB OTG driver. MiXplorer has one built in that allows writing to FAT32 & exFAT formatted drives ("Settings" -> "More Settings" -> "Enable OTG").

Running SMB server on Android without rooting is possible. But it's a huge hassle because Windows can only access a SMB share on port 445, which can't be opened on Android without rooting: https://android.stackexchange.com/questions/1705/using-smb-server-without-root-access

It's much easier to create a SMB share on PC and download files to your box with a SMB-client app, like MiXplorer or X-plore.

I use X-plore and I copy the files from the computer to my device directly from it, the other way around is a hassle as haojiezhu comments, what I always do is the following:

What you need:

1. You should have already the folders you need to access from your Mi Box marked as shared folders.
2. If you access your Windows 10 computer using your Microsoft account (usually Outlook or any other email address registered as a Microsoft account) you can use that email and your current password to access the computer from you Mi box otherwise you must enter whatever you use to access your computer.
3. You must use an static IP address on your Windows PC (i.e. 192.168.1.20 ) because X-plore doesn't auto recognize dynamic IPs and you must rescan the new IP again.
4. On your Mi box install and open X-plore and scroll down to "LAN"
5. The first time you use the app you must scan for servers, select "add server" -> "scan"
6. Once you see your computer's IP and name, click on it and then enter your email address and password where needed.
7. Click connect, you should see your shared folders.
8. To copy some files, (at first it's a little confusing), there are two panels, left and right panel, you must select first a panel as the target for your files, and the other panel will be the source.
9. First select the target panel, open the folder or the root of the SSD where you are going to copy the files, leave it open, then switch to the other panel where you are going to select the source folders/files, since you are going to copy FROM your computer, open LAN -> select your computer -> enter the username and password and then press and hold on the folder/file you want to copy from.
10. Select copy, and you will see a pop up window with the path of the target panel, press OK.

I guess you won't understand very well my instructions so don't hesitate to ask.