After plugging in the usb cable the Fujitsu scanner is found once:
uname -a scanimage --list-devices Linux raspberrypi 4.19.66-v7+ #1253 SMP Thu Aug 15 11:49:46 BST 2019 armv7l GNU/Linux device `fujitsu:ScanSnap iX500:10443' is a FUJITSU ScanSnap iX500 scanner
Doing it again, no scanner is found anymore:
scanimage --list-devices No scanners were identified. If you were expecting something different, check that the scanner is plugged in, turned on and detected by the sane-find-scanner tool (if appropriate). Please read the documentation which came with this software (README, FAQ, manpages).
This command finds the Fujtsu (and another unknown scanner) always:
sane-find-scanner -q found USB scanner (vendor=0x04c5 [Fujitsu], product=0x132b [ScanSnap iX500]) at libusb:001:010 found USB scanner (vendor=0x0424, product=0x7800) at libusb:001:004