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Re: No Sectors left of 120GB Drive

Message: 10 Date: Mon, 2 Oct 2006 15:32:14 -0700 From: "jdow" <jdow earthlink net> Subject: Re: No Sectors left of 120GB Drive

>>> >>> At 11:02 AM -0400 10/2/06, Tom Diehl wrote:
>>> >>>  ...
>>>> >>> >Will it let you make another primary partition?? I cannot remember
>>>> >>> >if you can have 2 extended partitions or not.
>>> >>>
>>> >>> I think one /can/, but it is contrary to specification (which you
>>> >>> showed below), which says there can be only one Extended partition
>>> >>> in a Basic partition table. There can certainly be both an Extended
>>> >>> and an LVM partition in a Basic partition table.
>> >>
>> >>but wouldn't that LVM correspond to just a primary partition?  thus
>> >>not violating the general rule of only one extended partition?
> >
> > Yes.

>In an abstract sense I'd love to know what limits a disk to only one
>extended partition other than legacy and sloppy code. I've had two
>extended partitions on other systems before. I think I did that with
>the 2.0.x kernel tree at one time, too. All that's needed is simply
>traversing the partition tables sensibly.

Having had to rebuild partition tables a few times from some over-enthusiastic misuse of Partition Magic, the answer is that:

a) partition table space is limited to four primary partition entries (this is 'legacy specification'); b) any of those entries can be a primary partition of an extended partition type, but you cannot have any primary partitions after that entry; c) a logical partition entry in an extended partition has a pointer to the start of next logical partition: the system follows the chain to find subsequent partitions; d) normally, the partition chain is linearly increasing, but you can have entries which jump 'back'. This completely defeats some operating systems, especially OS/2 (IIRC Windows does not mind this: I don't know about Linux and I have NO desire to find out the hard way!); and e) you may have had 2 extended partitions by creating an extended partition INSIDE the first extended partition. If you use a program which lets you change the table entries directly (partedit), this would be easy to do, since you are only changing a type byte, but I wouldn't do that unless I was testing to see if the OS could handle it, meaning, the drive had no important information on it, which naturally, was never the case, when I had to figure out what had gone wrong with my drives!

Partition Magic creates partitions in monotonic order notwithstanding that you may be jumping 'back' to fill a gap. It does NOT go back and change the pointers to push the new partition into the chain. If you are careful to move and expand partitions starting from the bottom and only create partitions in free space at the end, you can avoid this problem...maybe! I have no idea if the linux-based equivalent tools work the same way, but I strongly suspect they do due to the very nature of the beast. Backup the table first and backup your data.

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