Comment 8 for bug 879619

scanimage detects my SCX-4521F only the first time after connecting the USB.
SANE_DEBUG_DLL=3 SANE_DEBUG_XEROX_MFP=4 SANE_DEBUG_SNAPSCAN=128 scanimage -L
gives (among others):
 [sanei_debug] Setting debug level of xerox_mfp to 4.
 [xerox_mfp] sane_init: Xerox backend (build 12), version != null, authorize != null
 [xerox_mfp] sane_xerox_mfp_get_devices: 0x7fff65586338, 0
 [xerox_mfp] list_one_device: libusb:002:013
 [xerox_mfp] usb_dev_open: open 0x22e3970
 [xerox_mfp] :: dev_command(INQUIRY[0x12], 70)
 [xerox_mfp] dev_inquiry: found SAMSUNG/ORION
 [xerox_mfp] usb_dev_close: closing dev 0x22e3970
 [dll] load: searching backend `snapscan' in `/usr/lib/sane'
 [dll] load: dlopen()ing `/usr/lib/sane/libsane-snapscan.so.1'
 [dll] init: initializing backend `snapscan'
 [sanei_debug] Setting debug level of snapscan to 128.
 [snapscan] sane_snapscan_init
 [snapscan] sane_snapscan_init: Snapscan backend version 1.4.53
 [snapscan] sane_snapscan_get_devices (0x7fff65586338, 0)
 [dll] load: searching backend `hpaio' in `/usr/lib/sane'
 [dll] load: dlopen()ing `/usr/lib/sane/libsane-hpaio.so.1'
 [dll] init: initializing backend `hpaio'
 [dll] sane_get_devices: found 1 devices
 device `xerox_mfp:libusb:002:013' is a SAMSUNG ORION multi-function peripheral

then, when running scanimage -L for the second time, I get

 [sanei_debug] Setting debug level of xerox_mfp to 4.
 [xerox_mfp] sane_init: Xerox backend (build 12), version != null, authorize != n
ull
 [xerox_mfp] sane_xerox_mfp_get_devices: 0x7fff397db638, 0
 [xerox_mfp] list_one_device: libusb:002:013
 [xerox_mfp] usb_dev_open: open 0x2261970
 [xerox_mfp] :: dev_command(INQUIRY[0x12], 70)
 [xerox_mfp] usb_dev_request: sanei_usb_read_bulk: Error during device I/O
 [xerox_mfp] dev_command: dev_request: Error during device I/O
 [xerox_mfp] usb_dev_close: closing dev 0x2261970
 [xerox_mfp] list_one_device: dev_inquiry(libusb:002:013): Error during device I/O
 [dll] load: searching backend `snapscan' in `/usr/lib/sane'
 [dll] load: dlopen()ing `/usr/lib/sane/libsane-snapscan.so.1'
 [dll] init: initializing backend `snapscan'
 [sanei_debug] Setting debug level of snapscan to 128.
 [snapscan] sane_snapscan_init
 [snapscan] sane_snapscan_init: Snapscan backend version 1.4.53
 [snapscan] sane_snapscan_get_devices (0x7fff397db638, 0)
 [dll] sane_get_devices: found 0 devices

still can't scan..
I hope someone that knows what's going on finds this useful.