Want to backup an employee who just left the company, his mailbox is 20gb and archive are almost same size. How can I backup his mailbox including archives in full? Would b more helpful including OneDrive data, calendars data also.
Since most of our employees have E3 licenses, I've been using the Retention feature in the Security & Compliance Center to put a hold on mailboxes and OneDrives. Once it's placed under a hold you can then use eDiscovery against the held mailbox and export emails into a PST if needed.
Back it up where though? Office 365 is not a backup solution, neither is a file share replacement. If you want to keep the mailbox/onedrive data in place, use the hold/retention functionality as mentioned already. You will have to pay for (keeping) the license however.
Inactive mailboxes offer you a free alternative, but that only covers the Exchange part.