installed, each of them may - and probably will - assign different letters
to the same volumes. WinXP does not know what Vista has assigned - and vice
versa. Even a second copy of Vista does not know what letters the first
installation is using. That is a part of the reason why each primary
partition and logical drive should be assigned a volume label - a name -
that will be written to the hard disk and will be consistently available, no
matter which operating system is running.
Disk Management is the tool to use to assign or reassign "drive" letters -
except for the System Partition and Boot Volume. The only way to change
those is to run Setup, which means to reinstall Vista (or WinXP). When we
boot from the DVD to run Setup, it has no idea what letters have been
assigned in the past. It asks you where to install Vista, and then it
assigns C: to THAT volume, which becomes the Boot Volume for that
installation. Then Setup assigns other letters, starting with D:, to the
System Partition (if it is not also C:, the Boot Volume). If, as is usually
the case, we tell Setup to install Vista on the first primary partition on
the first hard drive, that partition will be assigned C: and will be both
System Partition and Boot Volume. But if we tell Setup to install Vista on
the 3rd logical drive in the extended partition on the second physical
drive, then THAT volume will become C: and Setup will have to assign a
different letter, probably D:, to that first partition on the first drive,
which will become the System Partition. Of course, when we next boot into
WinXP, it won't know what letters Vista has assigned, so the System
Partition will still be C: when in WinXP.
The only way we can at least partially control the assignment of letters is
to first boot into WinXP and use Disk Management to assign the letters we
want Vista to use. Then insert the Vista DVD and run Vista Setup from
within WinXP. This way, Vista Setup can see the letters that WinXP has
assigned and use those same letters in Vista.
I've used "WinXP" and "Vista", but the behavior is the same when installing
a second copy of WinXP or a second copy of Vista - or just about any other
second installation of Windows/Vista. Most of the comments apply when
installing only a single copy of either.
Don't rely on drive letters. Assign names to each volume and use Disk
Management to see which one currently is using which letter - and which is
the System and Boot volumes.
R. C. White, CPA
San Marcos, TX
Microsoft Windows MVP
(Running Windows Live Mail beta in Vista Ultimate x64)