One question:An OS only can boot from primary partition, not in an extended partition ?

grub 0.97

MBR is at the beginning of the medium (HD/USB/cd) that the BIOS has specified,not within a partition. Linux boot tools direct the MBR to consult the specified file (eg /boot/grub/menu.lst) within the required partition- generally, the partition you have just installed your OS in. If you are using multiple operating systems, then yes, this could be an extended partition.

Nobody can do **anything** from an extended partition......There are 2 types of real/useable partitions: primary and logical. An extended partition is only a pointer to the first logical partition**.

The Windows bootloader wants Windows to be on the first primary partition, but the typical Linux bootloader allows it to be on other primaries. (Not sure about logicals...)

Linux can be on either primary or logical partitions, with no restriction as to partition #1.


**Logical partitions are set up as a linked list. The first link is in the extended partition entry which appears in the main (MBR) partition table. Subsequent links are in additional extended partition entries in the linked partitions. An "extended partition" exists only as an entry in the partition table---i.e. there is no actual allocated space on the disk.

No problem with Linux :

I typically have 10 different Linux's installed on
e.g. sda6 ....... sda15.

.....

I has been many years since the last time I set up a multi-boot system with Windows booted from a logical partition (inside an extended partition). It was quite tricky to set up then and with newer versions of Windows I'm not certain it is still even possible. Anyway it is probably hard enough to be effectively impossible.

But booting Linux from a logical partition is no problem at all.

With Linux you can boot as many OS's(distros) as your brain is able to keep up with. What you do need is 1 primary partition, 1 extended partition, and as many as 16 logical partitions per HDD. If you want more than that you will have to go to LVM setup instead of partitions, but that's another story. I once read an article about a guy who had successfully installed & booted 100 distros on 1 machine. 6 is about all I can handle

As I recall it, when booting XP fom a logical partition you need an active primary (that does not have to be the first) containing the first stage boot files -- NTLDR, boot.ini, NTDETECT.COM (and another if using SCSI HDDs?). Wasn't hard (was all handled by the the installation process) once you got the idea and didn't take one of the installers' questions too literally.

If you look at (disassemble) the windows loader code you'll see it requires a primary partition - with the boot flag. Even checks that only one (primary) partition has a boot flag. (pre Windows 7 - I haven't bothered to look at that yet)
Old style *nix also require a primary "slice" - even OpenSolaris hadn't managed to fix this properly last I looked. As stated above Linux doesn't have any requirement for a primary, although I have seen distros fail to install if one wasn't available. This is badly designed installed code, not a Linux problem per se.