I read about making a backup of a Western Digital Mybook World edition, and how some of this backups fails at the moment of restore.
The main reason is those backups are made with partition imaging software (like Partition Magic or Partimage), and some guide includes instruction to save partition table and MBR, too. Right, but there are more data on disk that needs to be saved, and that cannot be recovered in any other way that do a raw image of a number of sector at the start of the disk.
Mybook disk configuration
In picture you can see schematic of the typical WD Mybook WE disk. At the start (in absolute sector 0) there are MBR and partition table (in blue). The partition table point to partitions, stored after some hundred of sector away (in yellow). The original partition table of Mybook place the first partition start on cylinder 4, that is over 48,000 sectors far away from MBR.
Why?
Because in that space there is some software that is vital for Mybook startup process: the bootloaders (in red in figure)!
So, if you want a really complete and working backup, you must save all this sectors in a raw file, using Linux command dd.
A rough procedure can be: use sfdisk to get the exact sector of start of partition sda1:
# sfdisk -d /dev/sda # partition table of /dev/sda unit: sectors /dev/sda1 : start= 48195, size= 5879790, Id=fd /dev/sda2 : start= 5927985, size= 208845, Id=fd /dev/sda3 : start= 6136830, size= 1975995, Id=fd
In this example sda1 starts at sector 48195. Well, when we save the MBR we use the command:
# dd if=/dev/sda of=sda-mbr bs=512 count=1
But this save only first sector. We need to save all the sector up to start of sda1 partition, so the command must be:
# dd if=/dev/sda of=sda-mbr bs=512 count=48195
Of course, change the number 48195 to match your disk configuration.
So, now we have the content of the entire space from MBR to start of sda1 partition, with all the bootloaders included.
Now the backup is really complete.
