Linux – Partition table not recognized by Linux kernel

fdiskkernellinuxpartedpartition

I experience relatively often that the partition table of a USB stick or SD card is suddenly no longer recognized by the kernel while (g)parted and fdisk still see it, as do other systems. I can even instruct gparted to do a fsck on one of the partitions but it fails of course because the device files let's say /dev/sdbX don't exist.

I'll attach the dmesg output:

[ 8771.136129] usb 1-5: new high-speed USB device number 4 using ehci_hcd
[ 8771.330322] Initializing USB Mass Storage driver...
[ 8771.330766] scsi4 : usb-storage 1-5:1.0
[ 8771.331108] usbcore: registered new interface driver usb-storage
[ 8771.331118] USB Mass Storage support registered.
[ 8772.329734] scsi 4:0:0:0: Direct-Access     Generic  STORAGE DEVICE   0207 PQ: 0 ANSI: 0
[ 8772.334359] sd 4:0:0:0: Attached scsi generic sg1 type 0
[ 8772.619619] sd 4:0:0:0: [sdb] 31586304 512-byte logical blocks: (16.1 GB/15.0 GiB)
[ 8772.620955] sd 4:0:0:0: [sdb] Write Protect is off
[ 8772.620971] sd 4:0:0:0: [sdb] Mode Sense: 0b 00 00 08
[ 8772.622303] sd 4:0:0:0: [sdb] No Caching mode page present
[ 8772.622317] sd 4:0:0:0: [sdb] Assuming drive cache: write through
[ 8772.629970] sd 4:0:0:0: [sdb] No Caching mode page present
[ 8772.629992] sd 4:0:0:0: [sdb] Assuming drive cache: write through
[ 8775.030231] sd 4:0:0:0: [sdb] Unhandled sense code
[ 8775.030240] sd 4:0:0:0: [sdb]  Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
[ 8775.030249] sd 4:0:0:0: [sdb]  Sense Key : Medium Error [current] 
[ 8775.030259] sd 4:0:0:0: [sdb]  Add. Sense: Data phase CRC error detected
[ 8775.030271] sd 4:0:0:0: [sdb] CDB: Read(10): 28 00 00 00 00 00 00 00 08 00
[ 8775.030291] end_request: I/O error, dev sdb, sector 0
[ 8775.030300] quiet_error: 30 callbacks suppressed
[ 8775.030306] Buffer I/O error on device sdb, logical block 0
[ 8775.033781] ldm_validate_partition_table(): Disk read failed.
[ 8775.033813] Dev sdb: unable to read RDB block 0
[ 8775.037147]  sdb: unable to read partition table
[ 8775.047170] sd 4:0:0:0: [sdb] No Caching mode page present
[ 8775.047185] sd 4:0:0:0: [sdb] Assuming drive cache: write through
[ 8775.047196] sd 4:0:0:0: [sdb] Attached SCSI removable disk

Here, on the other hand, is what parted has to say about the same disk, at the same time:

(parted) print                                                            
Model: Generic STORAGE DEVICE (scsi)
Disk /dev/sdb: 16.2GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End     Size    Type     File system  Flags
 1      4194kB  62.9MB  58.7MB  primary  fat16        lba
 2      62.9MB  16.2GB  16.1GB  primary  ext4

It's not only parted, even the older fdisk has no trouble with that partition table:

Command (m for help): p

Disk /dev/sdb: 16.2 GB, 16172187648 bytes
64 heads, 32 sectors/track, 15423 cylinders, total 31586304 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000dbfc6

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1            8192      122879       57344    c  W95 FAT32 (LBA)
/dev/sdb2          122880    31586303    15731712   83  Linux

I'm really clueless. It would be easy to say the partition table is corrupted but then why can gparted still read it without complaints (and there are none) or how can I reconstruct the partition table from what (g)parted miraculously found out?

Best Answer

For some reason your kernel fails to read the partition table:

[ 8775.030291] end_request: I/O error, dev sdb, sector 0
[ 8775.030300] quiet_error: 30 callbacks suppressed
[ 8775.030306] Buffer I/O error on device sdb, logical block 0
[ 8775.033781] ldm_validate_partition_table(): Disk read failed.

Thus, it can't create devices for partitions as it did not read the partition table. Later when you try to see the partition table with parted or fdisk the IO is performed successfully.

Try to use partprobe /dev/sdX when your kernel did not recognized the partitions at boot time.

man partprobe:

PARTPROBE(8)                                                         GNU Parted Manual                                                        PARTPROBE(8)

NAME
       partprobe - inform the OS of partition table changes

SYNOPSIS
       partprobe [-d] [-s] [devices...]

DESCRIPTION
       This manual page documents briefly the partprobe command.

       partprobe  is  a  program  that informs the operating system kernel of partition table changes, by requesting that the operating system re-read the
       partition table.
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