P. Burrows said:
"lets call them A and B." means its an example.
The fixed partitions never change letters, only on the two usb drives.
No support can be "can read and write to the drive but forgets the
letter"
Rescan Disks had no effect (ie, it didn't change the juggling around
behaviour of the drives)
Just a guess, but are you connecting the USB hard drives through a USB hub?
Or are you connecting them directly to USB ports in the computer case?
While it might be possible to enumerate different *single* devices connected
to the USB ports in the case (because they go to a specific port on one of
the USB controllers so that port can be uniquely identified), I doubt
enumeration works through a hub. You cannot guarantee which order the USB
devices are detected through a hub. It's not like old SCSI devices (that
don't rely on auto-negotiate) where you could dial in a specific ID number
on the device (and then had to make sure none of the others used it). I'm
not sure auto-negotiation of device ID on USB channels is guaranteed to
enumerate devices in the same order for device IDs.
Because they are USB-connected, they are not enumerated until detection.
That is after Windows loads, loads the USB device, and then checks what is
connected through it. I don't know that the USB driver will report the
volumed ID (not volume name) to the OS but might just simply represent it by
the device ID that got assigned to it when it got detected. So you might
need to switch which USB ports to which you connect so the drives gets
enumerated under the USB controller in the same order that you want them to
get enumerated, and not use a hub.
Of course, an update might fix the problem. After all,
http://support.microsoft.com/?id=830752 notes problems with USB-connected
CD-ROM drives that are supposedly fixed by the service pack. From another
post in this thread, it sounds like you haven't updated Windows XP to the
latest service pack. I've also seen USB problems based on which driver was
used. Some folks found problems with the vendor's driver and Microsoft's
USB driver worked okay (the ones included in Windows), but other have had to
get the latest motherboard driver package to get a newer USB driver. For
example, see
http://support.microsoft.com/?id=307070.
Do you have any mapped (network) drives? If so, see
http://support.microsoft.com/?id=297694. When assigning drive designators
to mapped drives, start at the end of the alphabet, or so high up that you
will never collide with newly added drives with their possibly numerous
partitions.
Are you connecting a USB 2.x external drive to a USB 1.1 port? While
hooking a USB 1.0/1 drive to a USB 2.x port should work (because USB 2.0 is
backwards compatible), it might not work the other way around (i.e., you
cannot manufacture hardware that is forward compatible with other hardware
or specifications that don't yet exist and which may significantly differ
from your old hardware or specifications).
I haven't waded through the SP-2 specifications as to what it fixed or
added, but I've heard that USB 2.0 got added in SP-2 (so you might have
problems with a USB 2.0 drive even if connected to a USB 2.0 capable port
but don't have SP-2 installed).