[syslinux] boot fails on some system

[Ady Ady]

> 
> On Thu, 13 Apr, 2017, 20:27:03 +0000, Ady Ady via Syslinux wrote:
> >
> > > I used syslinux to make an external usb hard drive booting up 
> "system rescue cd" from a FAT primary partition.
> > > The procedure I followed was setting on the boot flag on the FAT 
> partition and copying the content of the iso image to it and renaming 
> isolinux to syslinux wherever needed. I then unmounted the device and 
> ran "syslinux -d /syslinux/ -i /dev/sdxn" where sdxn is the appropriate 
> partition. It worked nicely on both laptops i have at hand.
> > >
> > > Then I wanted to try the same using an ext2 partition instead and 
> so using extlinux. For this, after creating an ext2 partition, moving 
> the boot flag to it and copying the live linux files, I copied the file 
> mbr.bin from the syslinux package to the hard drive mbr. Then I ran 
> "extlinux -i /mnt/mountpoint". This time booting up "system rescue cd" 
> failed on one of the two laptops. Afterwards even retrying to boot from 
> the FAT partition resulted with the same computer in failing. I hadn't 
> saved the initial mbr content, so couldn't restore it, hence mbr.bin 
> should still be in place and this might be what the failing laptop 
> doesn't like.
> > >
> >
> > (snip)
> >
> > How _exactly_ did you "copy" (again) the mbr.bin file (and why again)?
> > Which command did you use? Before you executed such command, was the
> > filesystem volume mounted or unmounted?
> >
> > Please post the exact command you used for formatting the FAT volume.
> > Please also post the command you used when creating the ext2 volume.
> >
> > If you repeat the _whole_ initial procedure you described (installing
> > mbr.bin, then using FAT and the 'syslinux' installer), can you boot
> > then?
> >
> > Regards,
> > Ady.
> 
> again? actually for the first test, i.e. booting from the FAT partition 
> I
> didn't put mbr.bin to the hard drive's mbr. I just executed the step as 
> I
> described.
> 
> I paid attention to mbr.bin only when I started to be interested in 
> bootting
> off an ext2/3 partition. And that's because it was mentionned in the 
> tutorial
> I looked at, which this one :
> http://www.syslinux.org/wiki/index.php?title=EXTLINUX, and I used the 
> exact
> same command i.e. "cat mbr.bin > /dev/sdx" where sdx is the master 
> device and
> while no partition of the device was mounted.
> 
> The FAT volume was formatted by gparted. For the ext2 partition I used
> '$mke2fs -t ext2 -L "sysrescd" /dev/sdxn'. I always use fdisk to toggle 
> the
> boot flag.
> 
> As i said this time the device is failing to boot on one of the two 
> laptops.
> Maybe I should have mentionned that it meant the other booted fine for 
> cases
> described in my initial email i.e. in the first test with the FAT 
> partition
> before paying attention to mbr.bin, in the case of the ext2 partition 
> after
> having modified the device's mbr by copying to it mbr.bin as described, 
> and
> in the case I reattempted to boot off the FAT partition while the mbr 
> of the
> device was modified.


Please next time reply to the Syslinux Mailing List email address.

I am aware that the EXTLINUX wiki page mentions 'cat' as a method to 
write mbr.bin. The reason for that wiki page to present such command is 
because the official documentation included to this day in the Syslinux 
distribution archives is also presenting the same command.

The suggested command to write to the MBR area is to first _unmount_ 
all the devices related to the physical device you want to write to, 
and then execute the 'dd' command as described at:

 http://www.syslinux.org/wiki/index.php/Mbr#Write 

Beware: use the correct "sdx" device for your case / OS; writing to 
'sda" if the device you are aiming at is a different one will cause you 
much more problems.

I would also add the 'sync' command, after dd'ing the mbr.bin file to 
the desired output device and/or after finishing any write operation to 
a portable device you are about to disconnect.

Having said that, there are many reasons for a system to fail to boot 
correctly, especially when using an external device. Some of them are:
_ mixing different versions of Syslinux-related files (although, since 
the device successfully boots another system, this doesn't seem to be 
the problem);
_ CHS geometry;
_ type of connection (e.g. USB 2.0 vs. 3.0)
_ booting as USB-Floppy (or some other mode) in the BIOS setting while 
having a different type of device (e.g. USB-HDD);
_ using a BIOS-based bootloader on a UEFI system while in UEFI mode, or 
some inverted situation...

There are many more alternative explanations / possibilities.

Saying "it fails to boot" doesn't really describe anything useful that 
would help in narrowing down the problem. We don't know what exactly 
you actually see on screen (blinking cursor perhaps?), or whether it 
reboots, or perhaps the connection/port/hardware is not "stable" 
enough... I'm not saying that one of these is the actual problem, but 
rather that we lack of enough information.

I would suggest repeating the procedure with the FAT fs, this time 
using dd to write the MBR.

Please note the correct device you are about to write to. Otherwise, 
you will overwrite the wrong device, making things even worse. Also 
note that the mbr.bin file shall be written to "sdX" (where "X" here 
represents the adequate device, and you should replace it with the 
adequate letter), and it is _not_ "sdx1" (i.e. do not use a partition 
device when writing the mbr.bin file).

Regards,
Ady.