I need a storage device in 3 different location. Each device will have 4TB of storage. At least 2TB should be a 'shared folder' working like Dropbox: when a user change a file in one location, the file should be updated instantly on the two other devices over internet (as fast as the network allows it of course). Each storage device will be connected to the local network and people using MAC/Linux will drop files to the 'external HD'.
I have thought about a NAS with a built in backup to another NAS in a different location (like Synology offers), but I don't know if that will work with more devices. Also thought about file storage, like glusterfs to replicate whole disk but it feels like going too far for such a job.
Could you recommend any software / setup for this? Thanks