problem with USB-CD drive

I wrote software for microcontroller with USB device port that presents 
itself as USB CD and includes ISO9660 image.

It works under windows - "CD" is detected and files readable
it works under MacOS - same
It mostly works under FreeBSD.


FreeBSD detects it properly


ugen1.5: <Digital Systems DS700> at usbus1
umass0 on uhub5
umass0: <Digital Systems DS700, class 0/0, rev 1.10/28.01, addr 5> on 
usbus1
umass0:  SCSI over Bulk-Only; quirks = 0x0100
umass0:3:0: Attached to scbus3
cd1 at umass-sim0 bus 0 scbus3 target 0 lun 0
cd1: <Digital DS700 Virtual CD 0128> Removable CD-ROM SCSI-2 device
cd1: Serial Number 220123456789
cd1: 1.000MB/s transfers
cd1: 16MB (8192 2048 byte sectors)
cd1: quirks=0x10<10_BYTE_ONLY>



i can read sectors by dd, by single (bs=2k) or multiple.
Other communication (vendor specific SCSI commands - used to 
configure/control the device with our software) - works properly.


If i make an image (first 34 sectors, device presents itself as 16MB
to prevent problems with some OSes but everything later are zeros) - it is 
good - did cmp with original image file.

i can do mdconfig, mount_cd9660 on this file and everything is fine.


BUT

mount_cd9660 /dev/cd1 /mnt

results in:

mount_cd9660: /dev/cd1: Invalid argument

There is no kernel messages.

How could i find out what is exactly a problem?