If I leave the setting "scan files": on network drives in "on access default processes poclicy" to unchecked, will usb flash drive be scan when inserted?
USB drives are local drives and not considered 'Network' drives. On-Access policies should be set within the local drives settings to handle USB drives.
Depending on the drive, it may or may not be listed as 'Removable.' Most Flash (thumb) drives are registered as removable, but external hard drives are commonly registered as non-removable.
For performance reasons, it may be advisable to Not scan inside archives, as the USB interface would be flooded with data flow (with USB 2.0 running at a substantially slower speed than SATA), which would be a duplicate scan during archive extraction. See the VSE 8.8 Best Practices guide regarding this.
However, Do Keep Scanning ON for both Reads and Writes, in order to potentially stop the transfer (both ways) from the USB device.
All of this is described throughout the Best Practices Guide.
I hope this is helpful.