Re: [gentoo-user] Trouble with backup harddisks

[Wols Lists <antlists@youngman.org.uk>]

On 30/04/20 10:32, tuxic@posteo.de wrote:
> Hi,
> 
> recently I switched from the old MBR-scheme to GPT on
> my new PC.
> 
> I have two external USB-harddisk, which were partioned/formatted with
> a MBR-scheme/MSDOS partition (but were never used to boot from. They are pure
> data containers).
> 
> When I connect these to my new PC, only the device is shown:
> /dev/sdb, /dev/sdc.
> 
> The kernel is configured (beside other things) as follows:
> 
> #
> # Partition Types
> #
> CONFIG_PARTITION_ADVANCED=y
> # CONFIG_ACORN_PARTITION is not set
> # CONFIG_AIX_PARTITION is not set
> # CONFIG_OSF_PARTITION is not set
> # CONFIG_AMIGA_PARTITION is not set
> # CONFIG_ATARI_PARTITION is not set
> # CONFIG_MAC_PARTITION is not set
> CONFIG_MSDOS_PARTITION=y
> # CONFIG_BSD_DISKLABEL is not set
> # CONFIG_MINIX_SUBPARTITION is not set
> # CONFIG_SOLARIS_X86_PARTITION is not set
> # CONFIG_UNIXWARE_DISKLABEL is not set
> # CONFIG_LDM_PARTITION is not set
> # CONFIG_SGI_PARTITION is not set
> # CONFIG_ULTRIX_PARTITION is not set
> # CONFIG_SUN_PARTITION is not set
> # CONFIG_KARMA_PARTITION is not set
> CONFIG_EFI_PARTITION=y
> # CONFIG_SYSV68_PARTITION is not set
> # CONFIG_CMDLINE_PARTITION is not set
> # end of Partition Types
> 
> CONFIG_BLOCK_COMPAT=y
> CONFIG_BLK_MQ_PCI=y
> CONFIG_BLK_MQ_VIRTIO=y
> CONFIG_BLK_PM=y
> 
> dmesg shows this:
> [14617.672363] usb 2-2: New USB device found, idVendor=1058, idProduct=25a2, bcdDevice=10.21
> [14617.672364] usb 2-2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
> [14617.672364] usb 2-2: Product: Elements 25A2
> [14617.672365] usb 2-2: Manufacturer: Western Digital
> [14617.672366] usb 2-2: SerialNumber: 575844314132383959393934
> [14617.681660] usb-storage 2-2:1.0: USB Mass Storage device detected
> [14617.681737] scsi host10: usb-storage 2-2:1.0
> [14618.725450] scsi 10:0:0:0: Direct-Access     WD       Elements 25A2    1021 PQ: 0 ANSI: 6
> [14618.725594] sd 10:0:0:0: Attached scsi generic sg1 type 0
> [14618.728090] sd 10:0:0:0: [sdb] Spinning up disk...
> [14619.748918] ...ready
> 
> I tried different USB ports in a desperate hope of success...
> ...no, same problem.
> 
> Interestingly fdisk shows the following:
> 
> host> sudo fdisk -l /dev/sdb
> Disk /dev/sdb: 931.49 GiB, 1000170586112 bytes, 1953458176 sectors
> Disk model: Elements 25A2   
> Units: sectors of 1 * 512 = 512 bytes
> Sector size (logical/physical): 512 bytes / 512 bytes
> I/O size (minimum/optimal): 512 bytes / 512 bytes

This could be the key. Sector sizes have been changing from 512 to 4096
over many years. If your kernel has been updated to expect/use 4096 byte
sectors, it might not be able to read the disk properly.

> Disklabel type: dos
> Disk identifier: 0x16f2a91f
> 
> Device     Boot Start        End    Sectors   Size Id Type
> /dev/sdb1           1 1953458175 1953458175 931.5G ee GPT
> 
> The type is shown as GPT...but the drive has a MSDOS partition table.
> 
> Reading my (old) internal harddrive with an external USB docking
> station is possible without any problems, though.
> 
> Unfortunatelu I have no space for 1T image of that drive -- otherwise
> I would have made an image copy and experiment with that.
> 
> So better ask, than sorry.... ;)
> 
> Is this fixable or did I lost my backups?
> 
Do you have access to an old kernel?

The other thing is try using gdisk (or that could be fdisk under another
name :-( But some partitioning schemes can write a GPT with protective
MBR - if you can find something that will take the MBR and write it as a
GPT that might help, too.

Cheers,
Wol