Back USB ports not working anymore

Hello everyone.

I'm new here. I tried to look into the discussions but I didn't find anything related to my problem.

Here's the situation:

I bought an external USB HD with power supply to do the backup of my NAS content. I bought it with the power supply so I can power it down when it's not used, to extend his life (my previous backup disk died after 1 year. Bad luck, I know, but if I can do some things to avoid it, it's better). So I connected it with a "smart" plug and tried to do some backups. Everything works, and I even saw that the backup jobs can also eject the device when they finish. Excellent: I can power my disk a few minutes before the scheduled backup jobs start, let the backup run, and then, a while (few hours) after the last job is finished and the USB drive is ejected set my "smart" plug to turn off the power.

I was testing my settings, and somehow it didn't work: the disk powered, but the jobs didn't run, with a "Destination cannot be accessed." error. I tried later to make them work again, and .. they worked. I sadly forgot that I set the "smart" plug to turn off at a certain time in the future, I started a backup job that lasted longer than expected, and then the "smart" plug turned the power off and the disk went offline. When I turned it back on, I got the following error "External storage 'My Book 25EE' might not have been safely ejected after last use. This device will be read-only. Plug it into its native operating system and repair it before it will be writeable."

I tried anyway to start the backup, and I got this error "Error backup job All: Destination is read-only or files/folders could not be created. Check if these files or folders meet the destination file system limitation.".

All this happened with the USB HD connected on the rear top USB port.

I then removed it from the USB port, connected it to my laptop, repaired it, connected it back on the same USB port and nothing happened.

I reformatted it, connected it back on the same USB port and nothing happened.

I recreated a new volume, connected it back on the same USB port and nothing happened.

I tried to connect an USB pen drive to the same USB port and nothing happened. I tried to change to the back lower USB port, and nothing happened as well. Then I tried the front USB port and it worked. I connected my USB HD to the front USB port and it was recognized.

I tried to restart the NAS but back USB ports don't recognize any USB device anymore.

So, my question is: Anybody has any idea on how to fix this problem? My last resort would be to reset the NAS to factory default....

if anyone has any idea, please let me know.

Thank you in advance.