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Old 06-30-2018, 11:38 AM   #1
cbuffer
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Registered: Feb 2012
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MBR


I have two harddrives on my desktop. sda has two older versions of mint. sdb has the latest mint. I want to remove those on sda and make sdb the first drive since it is the newest and best. It is already the drive which the bios is told to boot from. I've just done dd if=/dev/sda bs=512 count=1 | hexdump -C and the same for sdb. The dumps are different. I fear that removing everything on sda might leave me stranded. How can I ensure that the machine is really booting from the MBR on sdb?

TIA Ken
 
Old 06-30-2018, 12:19 PM   #2
jsbjsb001
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Quote:
Originally Posted by cbuffer View Post
I have two harddrives on my desktop. sda has two older versions of mint. sdb has the latest mint. I want to remove those on sda and make sdb the first drive since it is the newest and best. It is already the drive which the bios is told to boot from. I've just done dd if=/dev/sda bs=512 count=1 | hexdump -C and the same for sdb. The dumps are different. I fear that removing everything on sda might leave me stranded. How can I ensure that the machine is really booting from the MBR on sdb?

TIA Ken
You could just install GRUB on sdb from the system you would like to use as you're "primary" and/or only system. If you did say remove everything on sda and this left your system un-bootable; you could use the "GRUB Super Disk" run from a CD/USB and re-install GRUB from the system you would like to use as you're "primary" and/or only system.

https://www.supergrubdisk.org/
 
Old 06-30-2018, 12:42 PM   #3
hydrurga
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Detach sda physically. Try booting. If sdb boots ok, then you can muck around with sda at will.
 
Old 06-30-2018, 12:48 PM   #4
michaelk
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You have not posted enough information as to Mint versions or whether the computer is UEFI or legacy BIOS and how it is configured.

If the drives actually do have a legacy MBR then the boot code is the first 446 bytes and the rest is the partition table. Since you have two distributions on sda I would expect the partition tables to be different regardless if the drives are of different sizes. If the first 446 bytes are different then it might be due to the older versions using grub and the newer using grub2.

You could disable sda in the BIOS or disconnect its power cable. If the system boots like it usually does then I would have a high confidence that you are actually booting from sdb. It is possible that when installing Mint on sdb grub was actually installed to sda and that even though your selecting sdb in the BIOS it might automatically switch to sda if no boot loader was found on sdb.
 
Old 07-03-2018, 06:45 AM   #5
cbuffer
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Sorry for the delay in replying - fell and sprained my wrist.

Thank you for your suggestions, they've gone into the little black book I hope never to use again. I'm annoyed that I didn't think of disconnecting the drives to find the MBR. I had to spend most of yesterday one-handedly chasing multiple faults partly as a result of copying an old MBR to a backup drive in the past - how did I do that? -and made more difficult by the fact that my bios wouldn't recognize a flash drive with system rescue as a USB boot device without selecting it from the options in hard drives. Has never been a problem before, most irritating to have to keep going back to the bios to test alterations. However I now have an empty hdd to sort my photographs on and a network to link backup, desktop and our laptops and rsync playing nicely, so if my wrist proves fatal my wife will have all my passwords to spend my money.

Thank you all, Ken
 
  


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