If you see any errors in the Insync log (~/.config/Insync/out.txt) related to having too many files open, you can try the following:
- Open /etc/security/limits.conf for editing (using sudo)
- Add the following two lines to increase the hard and soft limit for the maximum number of open files to 65535
* soft nofile 65535
* hard nofile 65535
- Restart computer and insync.
On initial sync I ran into a ton of issues that were related to Insync opening too many files at once, causing it to be stuck at the same point forever. This solved the issue for me, so perhaps worth a try. After the initial sync you can probably set the ulimit back to default unless you frequently update large numbers of files at once.