Toggling Bootable Option Using cfdisk

paperbagperson · 03-16-2016, 02:35 PM

I'm playing around with cfdisk and basically here's what I did:

Code:

root@test:/# fdisk -l /dev/sda

Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders, total 976773168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00058c37

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048      487423      242688   fd  Linux raid autodetect
/dev/sda2          487424   957517823   478515200   fd  Linux raid autodetect

Code:

root@test:/# cfdisk /dev/sda

Toggled bootable, then wrote the new partition table to the disk.

Code:

root@test:/# fdisk -l /dev/sda

Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders, total 976773168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00058c37

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1            2048      487423      242688   fd  Linux raid autodetect
/dev/sda2          487424   957517823   478515200   fd  Linux raid autodetect

When I wrote the partition table it warned me that I could destroy data. I assume this is a generic warning in case I changed something like size. I'm still left wondering though:
1. What did I change about the disk?
2. Was any data destroyed?
3. Is there somewhere I can check to see the difference?

Thanks

rknichols · 03-16-2016, 07:10 PM

You changed the state of just one bit in the partition table in the MBR. It is a bit that is significant only to DOS/Windows boot loaders. No Linux boot loader cares about it. To see the difference you would have to have saved an image of the first sector of /dev/sda and compare that with the changed sector. You would see that the byte at decimal offset 446 changed from 0x80 to 0.