Port missing (Diecimila/MacBook)

Hi, new member here. First post.

I have an old Diecimila and a MacBook (non-pro, 2015 model, with one USB-C port to rule them all).

When I connect the Diecimila to USB, Arduino IDE will sees it and will upload sketches fine.

But, if I disconnect and reconnect the USB cable (or otherwise remove and restore power to the Diecimila), the IDE will not find the port again. It is not listed under Tools > Port.

I get this error if I hit upload: avrdude: ser_open(): can't open device "/dev/cu.usbserial-A5002sG0"

After I reboot the computer, it works again. This is a very inconvenient workaround.

If I’m missing any crucial info, let me know.

Thanks.

Some extra info:

I'm using Mac OS 10.13.2 High Sierra.

Also, it looks like the serial port is only found if Arduino is connected via USB while the computer is booting up.

Any ideas?

Edit: Just pulled an Uno from another project to test this and the same thing is happening. So I guess this issue is on the computer side.