On Sun, 2 Sep 2012, Alon Bar-Lev wrote: > > This shows the computer asking the drive to read 8 blocks starting at > > block 0. The drive (actually the JMicron USB interface, not the drive > > itself) returns an error code indicating that it thinks the command > > was not sent properly -- even though it was. > > > > I don't understand why the JMicron unit doesn't accept this command. > > It simply appears to be broken. Does it work if you plug it into a > > computer running Windows or Mac OS X? > > > > Alan Stern > > > > Hi, > > I don't know... I don't use these... and the disk is ext4... > > I have XP in qemu, I mapped the device and took usbmon (attached), so > you probably see plugin within Linux, the plugout, then windows takes > charge. > > I see the disk in device manager, but not in disk manager... don't know why. > > Maybe this will help... Yes, this shows what the problem is. It's a bug in the JMicron device. The usbmon trace shows that Windows uses READ FORMAT CAPACITIES and READ CAPACITY(10) commands to determine the size of the drive. These commands are not capable of reporting sizes larger than 2 TB, but they also say what the drive's block size is. They both say that the block size is 4096 bytes. Without your patch, Linux would also use READ CAPACITY(10). When it sees that the size is larger than 2 TB, it issues a READ CAPACITY(16) command, which _is_ capable of reporting the correct size of the drive. (With your patch, Linux tries READ CAPACITY(16) right away.) At any rate, it accepts the READ CAPACITY(16) results. But... READ CAPACITY(16) reports that the block size is 512 bytes, not 4096! Linux believes this value, and so when it wants to read 4096 bytes, it asks for 8 blocks. That's why the JMicron refused to carry out the read -- it realized that 8 blocks would be 32768 bytes, not 4096 bytes. Windows, on the other hand, believes the results from the other two commands and therefore knows that the block size is 4096. Therefore it is able to use the drive properly (although the usbmon trace doesn't show any place where Windows determines the drive's actual size). I can't see any reasonable way of fixing this problem. Even if Linux did settle for the READ CAPACITY(10) value, it would then think that the drive was only 2 TB. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
