Why reboot after fdisk of external drive?

[Robert Locke]

On Tue, 2008-03-11 at 13:04 +1100, Simon Slater wrote:
> 	G'day again, It must be my day for questions.  This more for filling in
> the blanks than problem solving.
> 
> 	I'm thinking of getting a Maxtor OneTouch IV 250GB external drive, so I
> googled other peoples experiences. On 2 sites (below) I found similar
> explanations of the steps to get one working under Linux.  Both say to
> reboot between fdisk and mkfs. Why?
> 
> 1/  http://www-personal.umich.edu/~hnarayan/maxtor-harddrive-linux.html
> 2/  http://www.totalpenguin.com/content/view/25/40/
> 

Well, the first link seems a little confused how partitioning works, but
the second one seemed a little better.

Anyway, I have not tried to use one of these "OneTouch" drives myself,
but the external USB drives I have purchased and used have been
substantially easier to work with.

First, most of these drives will probably come already partitioned and
formatted with either FAT32 or NTFS as filesystems.  If your intent (as
mine was) is to use the drive exclusively with Linux, I would prefer to
format it "ext3".

Anyway, if you do not need to "modify" the partition table, then don't.
But, when you modify a partition table on a drive that is already read
into the kernel's memory, when you "write" from fdisk, it will generate
a warning message suggesting that you reboot to get the kernel to
recognize your new partition table.  You can see what the partition
table on the physical drive is by running: "fdisk -l /dev/sdb" where you
would substitute the appropriate dev name based on what was recognized
when you attached the drive to your system (perhaps by reading dmesg
or /var/log/messages or the console output).  You can see what
partitions the kernel has recognized by running "cat /proc/partitions".
Now, as an alternative to "rebooting" after changing the table, you can
run the command "partprobe", which can force the kernel to "re-read the
partition table" from disk.  But you cannot run mkfs on a partition that
is not yet recognized by the kernel (meaning seen in /proc/partitions).
So if fdisk generates the warning (meaning the table has been previously
cached), then you need to either reboot or run "partprobe" before
running mkfs.

Hope that helps a little,

--Rob