https://COMPANYNAME.sharepoint.com/sites/SITENAME/_catalogs/masterpage/
The networked mapped location is this:
\\COMPANYNAME.sharepoint.com@ssl@443\sites\SITENAME\_catalogs\masterpage
Gotchas:
- Internet Explorer must have the site in the trusted intranet zone. Discuss with network administrator as this may involve editing group policy.
- You must sign into the site with Internet Explorer, and may have to allow IE to remember automatic login.
