How to "refresh" a mounted NFS folder from my router in Finder? I have a rt-n56u router with an external hard disk shared in NFS.

Sometimes this shared disk shows in Finder, sometimes not, and I need to reboot router AND mac yosemite - even if I can see it being shared in my router's admin interface - to make it available in my sidebar. Sometimes, I need to wait some minutes for it to show.

What can I do to "force", "refresh", "flush cache" or something like this for my hard disk to show again without all this hassle? I'm fine with command line.
Using the Connect to Server in Finder you can set the URL and force a refresh.  In Finder Go > Connect to Server or Command ⌘ + K From there you can add the URL to your server.  This Apple support article has further details.  I hope this helps.