To restore eM Client exactly as it was off an old hard-disk you normally do it to ways.
1). If you have been making regular eM Client backups either via the automatic periodic backup in Settings or manually via the Menu which by default go into your “Documents / eM Client” folder, then go to your old hard-disk and copy the eM Client dated backup.zip files from that folder to your new same “Documents / eM Client” folder
Then install “the same version of eM Client” either via the release history page or Windows store (depending on where you originally installed it from), and “Skip the setup wizard” and then restore your latest dated eM Client backup via “Menu / File / Restore”. It will then be exactly as it was.
2). If you haven’t been making regular eM Client backups, then you can restore all your complete data “via the user hidden database folder”.
To do that “go to your old hard-disk” and “copy the below eM Client complete hidden folder contents”.
(Hidden database folder Windows)
C:\Users%username%\AppData\Roaming\eM Client
(Hidden database folder Mac)
/Users/User Name/Library/Application Support/eM Client
Then “install the same version of eM Client” and open eM Client and “skip the setup” and then close eM Client.
Next go to your “current hidden database folder location” and “delete the folder contents”.
Then paste your old hard-disk same hidden folder contents into the new eM Client hidden folder.
Then reopen eM Client and it will be the same.
…
Lastly if you still don’t see all your old mail and old contacts, then they could have been in “Local Folders” at the bottom left of eM Client where you then just need to enable that in Settings.
To show Local Folders in V9, go to "Menu / Settings (Preferences) / General and enable Show local folders.
To show Local Folders in V10, go to “Menu / Settings (Preferences) / General / Folders” and enable Local Folders