On Tue, Oct 25, 2016 at 07:33:30AM -0400, Matt Price via talk wrote:
OK, so I did this
dd if=some.iso of=/dev/sdb
oops -- that's not the USB key! that's my internal m.2 drive!
The partition table is gone, but it used to contain 2 partitions, both of them in an LVM, one of them part of an extended logical volume that added space to /home on my overburdened main drive. I haven't lost much data (just the first 700mb were overwritten), and amazingly my laptop continues to run just fine -- even though lvscan reports a missing drive, apparently the data is still findable.
I'd like to restore the partition table but I don't know where the partition boundaries are, and in any case I don't know how to write a partition table (!). What tools should I use? Preferably without turning off my laptop, since I'm afraid it won't boot back up again!
Luckily gpt has two copies. One at the start of the disk and one at the end. Any gpt compatible partition tool should actually offer to copy the backup from the end of the device. For example: root@lsorensen-debian:~# cd /tmp/ root@lsorensen-debian:/tmp# dd if=/dev/zero of=disk.img bs=1M count=100 100+0 records in 100+0 records out 104857600 bytes (105 MB, 100 MiB) copied, 0.10877 s, 964 MB/s root@lsorensen-debian:/tmp# modprobe loop max_part=8 root@lsorensen-debian:/tmp# losetup /dev/loop0 disk.img root@lsorensen-debian:/tmp# gparted /dev/loop0 Created symlink /run/systemd/system/-.mount → /dev/null. Created symlink /run/systemd/system/boot-efi.mount → /dev/null. Created symlink /run/systemd/system/boot.mount → /dev/null. Created symlink /run/systemd/system/media-sf_Downloads.mount → /dev/null. Created symlink /run/systemd/system/run-user-1000.mount → /dev/null. Created symlink /run/systemd/system/tmp.mount → /dev/null. ====================== libparted : 3.2 ====================== /dev/loop0: unrecognised disk label Removed /run/systemd/system/-.mount. Removed /run/systemd/system/boot-efi.mount. Removed /run/systemd/system/boot.mount. Removed /run/systemd/system/media-sf_Downloads.mount. Removed /run/systemd/system/run-user-1000.mount. Removed /run/systemd/system/tmp.mount. root@lsorensen-debian:/tmp# gdisk /dev/loop0 GPT fdisk (gdisk) version 1.0.1 Partition table scan: MBR: not present BSD: not present APM: not present GPT: not present Creating new GPT entries. Command (? for help): n Partition number (1-128, default 1): First sector (34-204766, default = 2048) or {+-}size{KMGTP}: Last sector (2048-204766, default = 204766) or {+-}size{KMGTP}: Current type is 'Linux filesystem' Hex code or GUID (L to show codes, Enter = 8300): Changed type of partition to 'Linux filesystem' Command (? for help): w Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING PARTITIONS!! Do you want to proceed? (Y/N): y OK; writing new GUID partition table (GPT) to /dev/loop0. The operation has completed successfully. root@lsorensen-debian:/tmp# gdisk -l /dev/loop0 GPT fdisk (gdisk) version 1.0.1 Partition table scan: MBR: protective BSD: not present APM: not present GPT: present Found valid GPT with protective MBR; using GPT. Disk /dev/loop0: 204800 sectors, 100.0 MiB Logical sector size: 512 bytes Disk identifier (GUID): 5946A311-C895-4DAE-94CF-9C96EB8E14C1 Partition table holds up to 128 entries First usable sector is 34, last usable sector is 204766 Partitions will be aligned on 2048-sector boundaries Total free space is 2014 sectors (1007.0 KiB) Number Start (sector) End (sector) Size Code Name 1 2048 204766 99.0 MiB 8300 Linux filesystem root@lsorensen-debian:/tmp# dd if=/dev/zero bs=1M count=5 > /dev/loop0 5+0 records in 5+0 records out 5242880 bytes (5.2 MB, 5.0 MiB) copied, 0.0397727 s, 132 MB/s root@lsorensen-debian:/tmp# gdisk /dev/loop0 GPT fdisk (gdisk) version 1.0.1 Caution: invalid main GPT header, but valid backup; regenerating main header from backup! Caution! After loading partitions, the CRC doesn't check out! Warning! Main partition table CRC mismatch! Loaded backup partition table instead of main partition table! Warning! One or more CRCs don't match. You should repair the disk! Partition table scan: MBR: not present BSD: not present APM: not present GPT: damaged Found invalid MBR and corrupt GPT. What do you want to do? (Using the GPT MAY permit recovery of GPT data.) 1 - Use current GPT 2 - Create blank GPT Your answer: 1 Command (? for help): w Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING PARTITIONS!! Do you want to proceed? (Y/N): y OK; writing new GUID partition table (GPT) to /dev/loop0. The operation has completed successfully. root@lsorensen-debian:/tmp# gdisk -l /dev/loop0 GPT fdisk (gdisk) version 1.0.1 Partition table scan: MBR: protective BSD: not present APM: not present GPT: present Found valid GPT with protective MBR; using GPT. Disk /dev/loop0: 204800 sectors, 100.0 MiB Logical sector size: 512 bytes Disk identifier (GUID): 5946A311-C895-4DAE-94CF-9C96EB8E14C1 Partition table holds up to 128 entries First usable sector is 34, last usable sector is 204766 Partitions will be aligned on 2048-sector boundaries Total free space is 2014 sectors (1007.0 KiB) Number Start (sector) End (sector) Size Code Name 1 2048 204766 99.0 MiB 8300 Linux filesystem So seems simple enough to recover the partition table from the backup. -- Len Sorensen