When syncing: Replace Server Data or Replace 2Do data?
The first time you sync 2Do on a device by whichever sync method you have chosen, you will be asked to specify if you wish to "Replace Server Data” or "Replace 2Do Data.” In this context, "Server” refers to the service on the other side of the sync process (iCloud, Dropbox, ToodleDo, or a CalDAV server); and "2Do” refers to the app installed on your device.
If you have tasks in 2Do and have never performed a sync, then the server-side is empty, and you MUST choose "Replace Server Data” for your first sync. This will push a copy of your 2Do database from your device up to your chosen sync service; what was empty on the server will now contain all the tasks from your mobile device. If you make a mistake and choose "Replace 2Do Data”, what’s on the server (nothing) will be pushed down to your device, and your tasks will be wiped from the app. If that happens, you can recover and start again – please read the topic entitled Backup / Restore.
If you do not have tasks in 2Do, and you have tasks on the server-side (because you’ve been using the toodledo.com website, or because you’ve already been syncing another mobile device and you now want those tasks to appear on another mobile device), you MUST choose "Replace 2Do Data” for your first sync. This will pull a copy of your 2Do database from the server down to 2Do on your device. If you make a mistake and choose "Replace Server Data”, what’s on your device (nothing) will overwrite what’s on the server (everything), and this is potentially NOT recoverable. Please take great care in this situation.
Once you have a successful sync, the app should reset to "Sync Normally”, but you might wish to double-check. In a normal sync, changes are pushed in both directions: from the app up to the server, and from the server down to the app. This allows you to make changes to your tasks on two or more devices, and to have those changes reflected on your other devices.