Use Client side files when kicking off sync

I have 2TB of data set up as read-only external storage on NextCloud.

I have a local copy of the 2TB of data, it is a few weeks old, so there is a difference of maybe 100Gb, other than that, it’s identical.
I have (a) copied that data to the appropriate local folder, and then (b) activated sync to that folder.

However, it appears that NextCloud is ignoring all the content that is already there, and re-downloading all files. It’s indicating a 9 day download time at this point, where the 100Gb should be possible in an afternoon.

Is is possible to change some setting (I’m not afraid of the command line) to make NextCloud use the existing files? I note that Dropbox handles this exact scenario with no problems.

Yes, that is a problem that everything must completely be synced through the client. It’s a missing feature, here is the feature request:

It was a long-standing open feature request in ownCloud, they partly solved it (if mtime and size are the same). I don’t know if that can be backported easily.

I’ve since had a look around and it looks like SyncThing is a better bet. I’m trying to sync a filesystem to an offsite failover server over the public internet via VPN.

It’s a cool solution, if you only want to sync things quickly and without server. In case you want to share things or access through Nextcloud, you can as well use such a sync folder as external storage on Nextcloud (the syncthing folder as “external” (to NC) storage on a local folder of the server).