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From: Nick Naym on 16 Jul 2010 22:23
1. What determines the order in which one's external hard drives (I have
more than one) appear in DU?

2. Is there any way that an external hard drive that I had GUID partitioned
could (through some software corruption, abuse, Act of God) suddenly become
an Apple Partition Map hard drive -- and do so without having its data wiped
clean?

--
iMac (27", 3.06 GHz Intel Core 2 Duo, 4 GB RAM, 1 TB HDD) � OS X (10.6.3)

From: David Empson on 17 Jul 2010 03:03
Nick Naym <nicknaym@_remove_this_gmail.com.invalid> wrote:

> 1. What determines the order in which one's external hard drives (I have
> more than one) appear in DU?

The order in which the system has iterated them in its list of hard
drives (/dev/disk0, /dev/disk1, etc.). Your startup drive is probably
the top one, and the rest will depend on the order in which the kernel
drivers iterate internal drives, USB and Firewire drives.

For two USB drives, there is a logical address assigned to each drive,
which will be consistent as long as everything is connected the same way
at startup, so the drives should always appear in the same order.

I'm less certain what will happen with Firewire, as it may depend on the
order in which devices are powered on, which device ends up as the
Firewire master, and the order in which other devices connect to the bus
and are allocated addresses. I've never looked closely enough at two
connected Firewire drives to see if they can change logical device
numbers in Mac OS X.

> 2. Is there any way that an external hard drive that I had GUID partitioned
> could (through some software corruption, abuse, Act of God) suddenly become
> an Apple Partition Map hard drive -- and do so without having its data wiped
> clean?

Absolutely not, short of software which deliberately overwrote the
partition map so that the partitions ended up in exactly the same
places. GPT has extra "buffer zones" between partitions, which would
have to be set up as small reserved partitions in APM. There may be
other technical issues which prevent direct conversion of GPT to APM.

Direct conversion of APM to GPT is not possible due to the lack of
buffer zones and probably not enough space for the partition table.

--
David Empson
dempson(a)actrix.gen.nz
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