Hello, (sorry for the long post, but I want to be as informative as possible)
I have now spent a full day trying to troubleshoot this issue and reading (I think) all of the posts regarding issues with blinkning L LED, unexpected reset and sketch restarting with the error message: "resource busy" (see full error message below).
So here is the issue:
I just bought the Arduino Uno starter Kit. Plugged it in to my MacBook Pro 15" (2017) and started following the guide book. When I came to the first project involving programming I used the code and the project worked fine, except that the sketch constantly restarted after 4-5 seconds, when this happens, the L LED goes from fully lit, to blinking 4 times, then turns off and then starts to fully lit again. Just as hitting the reset-button on the board. Also when uploading a sketch to the Arduino Uno, I need to press the Upload-button several times until it actually uploads. This is because I get this error message:
Arduino: 1.8.12 (Mac OS X), Board: "Arduino Uno"
Sketch uses 444 bytes (1%) of program storage space. Maximum is 32256 bytes.
Global variables use 9 bytes (0%) of dynamic memory, leaving 2039 bytes for local variables. Maximum is 2048 bytes.
avrdude: ser_open(): can't open device "/dev/cu.usbmodem145101": Resource busy
Problem uploading to board. See http://www.arduino.cc/en/Guide/Troubleshooting#upload for suggestions.
This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences.
When working on this issue I tried uploading a fullt blank sketch, with exactly the same result and also a sketch which just turns a LED. When the L LED starts to blink, the connected red LED turns off and then turns on again when the L LED starts to light up again. So this is not a coding issue.
I tried reinstalling the IDE completely, with the same issue persisting. So not a software at the MacBook.
The Arduino is connected to a USB-A to USB-C adaptor and then to the MacBook. I have two different adapters and tried both of them in all the ports with the same issue. So this is where I think the problem lies, power! Is the continues restarts and L LED blinking a sign of "not enough power"?
To figure this out, I tried connecting my Arduino to a normal power outlet and it works just fine! WIHOO!
BUT, when I also plug the USB 2.0 between my adapter and MacBook to the Arduino (to be able to upload sketches), at the same time as the electrical outlet is connected to the Arduino. The problem is back, still the L LED blinkning, the sketch restarts and the "Resource busy" error message.
I have now ordered a new cable which is USB-C to USB 2.0 so I don't need the adapter, and hopefully this will work. But is there anyone else that might recommend another solution, of which I have not already tried?
Thanks!