Writing to SharepPoint Online mapped network drive

I'm trying to create a custom master page template in a SharePoint Online environment. I'm using the Design manager to upload the design files. I've mapped the network drive like the page described and can open and view the files, but I cannot upload files to the location. Every time I try I get the following error:

Error 0x800700E0: Access Denied. Before opening files in this location, you must first add the website to your trusted sites list, browse to the website, and select the option to login automatically.

I've added the site to the trusted sites list, as well as selected the option to login automatically. The WebClient service is also running.

How can I upload files to this location?

The only explanation I can think of is that I am logged into windows on a Microsoft account, and I use a different Microsoft account for SharePoint. I can map the network drive fine, but when I try and map it with the option "Connect using different credentials", and I use my SharePoint Online account, I get the same access denied error.

Thanks

Check permissions for the document library/folder in which you're trying to write files. Folders like _Layout which resides at root level sometimes do not allow access of write. Global administrators have full access to these folders but tenant or site collection administrators may not have its access.... For example try opening this link in browser https://yoursharepointsite.com/_layouts/15/fonts this is where font files are like Arial.ttf or Comic sans.ttf So if you want to add new font to your sharepoint online themes you'll have to add files here.

Do this open SP Designer -> open main site -> browse left side menu for your folder and try copying something. If you can copy files there you should be able to copy through your mapped drive.

Also when you mapped drive in Windows Explorer didn't it ask for credentials, where you had to give in your Office365 login email then it can't be an issue of your windows credentials messing up with anything.