previous I/O error to superblock detected on my USB flash disk partition

Not sure if there is way to stop monitoring the status of a USB flash disk partition. 

You can check following article to see if that is useful:

https://community.fortinet.com/t5/FortiGate/Troubleshooting-Tip-How-to-clear-short-of-flash-space-in/ta-p/274293

https://community.fortinet.com/t5/FortiGate/Technical-Tip-Standard-procedure-to-format-a-FortiGate-Log-Disk/ta-p/190473

From the security perspective, the only possibility to turn off USB "monitoring" would be to disable auto-install options on FortiGate.

There is no way to permanently disable it/shut it down. Also, we strongly recommend not to plug in anything in the USB port for more time than necessary. The port cannot be use to expand the storage or save logs. The config backup should be done on dedicated servers or Fortimanager. A permanently plugged in USB drive will eventually fail.

TestFG # config system auto-install

TestFG (auto-install) # show full
config system auto-install
set auto-install-config enable
set auto-install-image enable
set default-config-file "fgt_system.conf"
set default-image-file "image.out"
end