I got it to work, but I didn’t really had to do much.

All of my problems were really about nfs permissions and were on the NAS config side.

Once those were fixed and I could mount without any problem, I didn’t even need any of the sync code on emulationstation. I have an entry on /etc/fstab and it just loads fine every time…

192.168.1.100:/volume1/games/roms /home/pi/RetroPie/roms nfs defaults 0 0

In my case I’m linking the whole roms folder, which I initially copied over to my NAS as it comes by default on RetroPie.

I like the usb idea though for whenever there’s no access to nfs. Even if it’s not a USB drive, the SD card could be just fine. Just having a folder on the SD following same roms folder structure, say /home/pi/RetroPie/romslocal, but just with a couple of cool roms, and have emulationstation use it when nfs mount fails. Not sure how I could get romslocal be “mounted” on /home/pi/RetroPie/roms, since it’s not really a device.

Another option I’m thinking would be to have two different copies of the es_systems.cfg file, es_systems.cfg.nfs and es_systems.cfg.local, and depending on whether nfs is available or not, copy the right one to es_systems.cfg on the emulationstation script. But yeah…two config files to maintain.

In your case, and since you have all that sync code on emulationstation anyway, probably it won’t be too hard for you to add a couple of lines after all the nfs mount retries to try the alternate USB drive mount…