How to Sync Existing Windows File Shares to Microsoft/Office 365 using SharePoint and OneDrive

_{Create a link between your existing on-premise file shares to SharePoint to have access to them from OneDrive/365 services regardless of your network connection (this does require you to have active internet to work when accessing files, unless you store a copy of all OneDrive folders on all of your devices)}

Purpose
The purpose of this article is to show/guide you through setting up an existing file servers shares to be connected/synced to OneDrive. This will allow users remote access and utilization across Microsoft 365 applications using SharePoint, no 3rd party services required to function. This solution requires no downtime and can be performed during business hours. However, the initial sync will impact internet upload speeds; so be sure to understand the impact of slow upload bandwidth to the organization prior to completing step 7 (you may also wish to adjust upload rate settings from the one drive application on the server, OneDrive -> Settings -> Network if desired during initial upload and then adjust afterward)

It is recommended that you plan out folder to SharePoint document library structure prior to creation to avoid re-work. This process will create a "Top Level Container" (called "Document Library" in SharePoint site) and all of your folder shares will be nested with-in it. Without reviewing your situation, needs and intended goals I cannot recommend what may work best for you. With that said, my general "go-to" is to base the "top level container" design plan around departments or existing share groups" ie, General, HR, Support, Admin... 

Requirements/Assumptions
_{Each of the SharePoint/365 Steps are completed using the an account with the correct administrative permissions to perform the actions.}

• Windows Server 2016+
• OneDrive is installed on Server/Share sources
• Dedicated/General Account For syncing (you decide what is best for you and your companies policy)
• Existing File Shares or Folders for linking/syncing
• Correct Administrative rights for Microsoft 365
• Correct Administrative rights for the server
• This is technically optional since you can complete this task using mklink CMD commands, however to follow the guide you will need to have Shell Extension installed on the server (to avoid environment specific issues when using mklink commands) it can be downloaded here (Link Shell extension), it can be uninstalled after configuration if desired.

The Process
_{Each of the SharePoint/365 Steps are completed using the 365 Syncing Account, all computer/server side are completed using a domain admin account.}

1. Create SharePoint Site
   1. Create SharePoint Document Site (or using existing, whatever makes senses for your project
   2. Assign users to site (this can be completed at the end, however I prefer to not have to go back after the fact)
      1. Users will receive an email of the new site membership
   3. Bookmark/Save the site URL
2. Create Document Library for Each Shared Folder or Group. Published/linked shares being added to SharePoint will be nested with-in this Document Library
   
3. Click “Sync” link at the top the newly created document library created (this will create a folder in OneDrive root folder that will be used to connect the on-prem folder share to SharePoint document Library)
   
4. On the server: right click the shared folder (or desired sync folder) and select “Pick Link Source” (must have link shell extension installed
   
5. Go to the folder created by the “Sync” link process in step 3, open it up.
6. Right click in the folder, select “Drop As… -> Junction”
   
7. A “Short-Cut” will be created, and you will see the “Sync” status icon and the files with in the share will start syncing to SharePoint/OneDrive
   
8. Have users create shortcut to new folder or folders. To do this, have users log into SharePoint using the sites direct link (that you bookmarked earlier; or wait 1-2hrs for the SharePoint link to be added to their profiles)
9. Once logged into the site, open the desired Document Library, open the desired subfolder (or don’t) and click the “Add Shortcut to OneDrive” link from the top bar
   

The user/s will now have access to the on-premises shared folder from anywhere that has access to SharePoint or OneDrive. This includes just about all Microsoft products that the user is signed into as well as web interfaces.