Comment 78 for bug 581925

Now it seems clear to me that if you request 17 blocks, many if not most of firmwares will check only the MSB and make it 16 blocks, what means that just sector 0 will be checked, which is non-empty solely in case of rewritable DVD formatted to a filesystem such as fat32/ext2, all ISO and UDF discs have it blank and the most important sector will be missed. And the problem with incorrect cd_media variable settings causes all disks to be checked, checked (depending on the firmware used) in wrong way.

The number of blocks in the read operation is rather a sensitive value, with my LG drive I observed that if you overwrite its cache in one ioctl it is gone from BIOS devices until next cold start.