arduino nano, the driver is not showing up in Port assignments. It has worked with this operating system before. I’ve messed something up. “OS X EI Capitan ver. 10.11.3 I have check the posts but the serial port is a no show
If the port showed in earlier versions of OS X, but never showed after upgrading to El Capitan, then you probably used an unsigned driver, which is no longer allowed by Apple's new System Integrity Protection (a.k.a. "Rootless"; more details in a WWDC session).
Any unsigned drivers will have been disabled during the upgrade process. Even more, SIP does not allow any non-Apple files in
/usr (except for
/usr/local), so anything you had put in there will have been moved away as well. To see what has been disabled:
ls -l "/Incompatible Software"
ls -lR /Library/SystemMigration/History/Migration-*/QuarantineRoot/*
To get it working again: find a signed driver, or Switch off SIP.