Can't Upload to Board?

Hello, fellow makers;

I've been fooling around with arduino a lot lately and have gotten quite a few projects done, so I'm not a newbie to the coding. However, when I tried to upload my latest project to the arduino, I've gotten nothing but the following error message:

avrdude: ser_open(): can't open device "/dev/cu.usbmodem621": Resource busy
ioctl("TIOCMGET"): Inappropriate ioctl for device
Problem uploading to board. See for suggestions.

So, I'm stumped with this one. Can somebody suggest what to do next?

I'm on Arduino 1.6.6 on a Mac, and the board I'm working with is an Uno.

Wrong port selected? Something else using the port?

What OS version is it? Did you recently upgrade MacOS? (ie, since it last worked) I've heard negative things about recent versions of MacOS wrt serial port handling :-/