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c00l.wave
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Joined: 24 Aug 2003
Posts: 213

PostPosted: Sun Sep 08, 2013 9:36 pm    Post subject: GRUB 0.97 misreading hybrid MBR Reply with quote

I installed a 3TB hard drive yesterday. In order to use the full 3TB, I used GPT and added a hybrid MBR as I did many times before on other boxes. However, this one behaves a bit whacky, as GRUB sees just garbage:

Code:
grub> root (hd0,
 Possible partitions are:
   Partition num: 0,  Filesystem type unknown, partition type 0x83
   Partition num: 1,  Filesystem type unknown, partition type 0x83
   Partition num: 2,  Filesystem type unknown, partition type 0x83
   Partition num: 3,  Filesystem type unknown, partition type 0x83
   Partition num: 4,  Filesystem type unknown, partition type 0x83
   Partition num: 5,
Error 18: Selected cylinder exceeds maximum supported by BIOS


Code:
# gdisk /dev/sda
GPT fdisk (gdisk) version 0.8.6

Partition table scan:
  MBR: hybrid
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with hybrid MBR; using GPT.

Command (? for help): p
Disk /dev/sda: 5860533168 sectors, 2.7 TiB
Logical sector size: 512 bytes
Disk identifier (GUID): 0D0B5AC5-FA29-4853-B44A-8CD9C7D8F755
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 5860533134
Partitions will be aligned on 2048-sector boundaries
Total free space is 8158 sectors (4.0 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            8192          212991   100.0 MiB   8300  boot
   2          212992       136527871   65.0 GiB    FD00  raid_rootfs
   3       136527872       153305087   8.0 GiB     8200  swap
   4       153305088      1201881087   500.0 GiB   0700  windows
   5      1201881088      2250457087   500.0 GiB   8E00  lvm
   6      2250457088      5860533134   1.7 TiB     FD00  raid_lvm

Command (? for help): r

Recovery/transformation command (? for help): o

Disk size is 5860533168 sectors (2.7 TiB)
MBR disk identifier: 0x275B40BB
MBR partitions:

Number  Boot  Start Sector   End Sector   Status      Code
   1                     1         8191   primary     0xEE
   2                  8192       212991   primary     0x83
   3      *      153305088   1201881087   primary     0x07


Any idea what's going on? I spent the last 5 hours or so installing Windows 7 to sda4 (MBR: sda3) and it worked fine. Not sure if that has anything to do with GRUB being unable to read the MBR, I forgot to check if it could read the MBR correctly before I started the Windows install. I've setup 4 other machines before using the same method (all Linux-only) and never encountered such a problem.

And unless GRUB 2 has been greatly improved on Gentoo since last year, I'm not willing to spend another couple of hours trying to figure out how to get it installing to disk...
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DONAHUE
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Joined: 09 Dec 2006
Posts: 6254
Location: Goose Creek SC

PostPosted: Sun Sep 08, 2013 11:47 pm    Post subject: Reply with quote

http://www.rodsbooks.com/gdisk/hybrid.html

legacy grub still works well with protective MBR on gpt and with uefi on gpt
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c00l.wave
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PostPosted: Mon Sep 09, 2013 8:17 am    Post subject: Reply with quote

I tried again, this time asserting I see GPT partition numbers, not MBR, and it worked (hd0,3 for sda4 instead of hd0,2 for sda3@MBR). I'm just wondering: I read that GRUB Legacy is able to read GPT only with a patch. I suppose that is included in Gentoo? I still don't get why it worked on other systems by reading the MBR, though.

If it's just a hacky patch I'm not too surprised the partition types are all off. GRUB should have displayed 83, FD, 82, 07, 8E, FD in my case but shows all partitions as 83 instead. That's not too helpful when identifying partitions. :(

@Protective MBR: I have to use a hybrid MBR as I'm going to replace all older disks (2x 750GB, 1x 300GB) with 2x 3TB and maybe a SSD later on (that way, there's no pure MBR left to boot from except the optional SSD that I don't have yet). Windows can only install to GPT if the system is booting via UEFI which I don't have (mainboard is from 2007), so I have to rely on a hybrid MBR to get dual-boot. But good to know GRUB (on Gentoo) can handle GPT directly so I won't have to use hybrid MBRs in the future on Linux-only machines.
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DONAHUE
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PostPosted: Mon Sep 09, 2013 6:16 pm    Post subject: Reply with quote

Disclaimer: gpt disk but with protective mbr:
Either entry works fine in grub.conf:
Quote:
default 0
timeout 5
#splashimage=(hd0,0)/boot/grub/splash.xpm.gz

title Gentoo 3.10.9
root (hd0,0)
kernel /boot/vmlinuz-3.10.9-gentoo root=/dev/sdf1

title Gentoo 3.10.9 PARTUUID
root (hd0,0)
kernel /boot/vmlinuz-3.10.9-gentoo root=PARTUUID=c14c603c-c0fa-48d4-9124-36bd18e43756


PARTUUID available from blkid or gdisk or in /dev/disk/by-partuuid

I have an idle disk, will give building a hybrid mbr a shot, never tried it.
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