What is maximum number of drives? Or maybe...

I have just encountered a problem, which I didn't have before...

I've just upgraded from an old PC, running Windows 2k Pro (sp4), to a
new one running XP Pro (sp2). (I have built both PCs, just as I have
always done.) My configuration is:

----+-------+-------+---+----------------------
Drv | OLD PC| NEW PC| # | description
----+-------+-------+---+----------------------
A: | x | x | - | floppy
B: | - | - | - | doesn't exist
C: | x(10) | x(35) | 0 | OS partition (active)
D: | x (5) | x (8) | 0 | comm partition
E: | x (5) | x (8) | 0 | graphics partition
F: | x (5) | x (8) | 0 | word proc partition
G: | x (5) | x (8) | 0 | database partition
H: | x (5) | x (8) | 0 | various data
I: | x | x | - | CD / DVD-ROM drive
J: | x | x | - | DVD drive
K: | x | x(10) | 1 | OS partition (active)
L: | x(320)| ????? | 2 | misc
M: | | x (5) | 1 | comm partition
N: | | x (5) | 1 | graphics partition
O: | | x (5) | 1 | word proc partition
P: | | x (5) | 1 | database partition
Q: | | x (5) | 1 | various data
R: | | x | - | card read 1st drive
S: | | x | - | card read 2nd drive
T: | | x | - | card read 3rd drive
U: | | x | - | card read 4th drive
V: | | x(160)| 3 | misc

Right now, disk 1 in the new PC is my disk 0 from my old PC, while I
continue to migrate from the old to the new. *Normally*, I would only
have one, perhaps two partitions on disk 1.

Hard Disk 0 = 80gb
Hard Disk 1 = 40gb right now, but varies (win 2k hdd 0)
Hard Disk 2 = 320bg right now, but varies
Hard Disk 3 = 160gb right now, but varies

Notes:
------
1.) Disks 0, 1 & 2 are in removable cartridge/chassis.
2.) Disk 3 is an external unit with USB interface.
3.) All hard disks are IDE.
4.) Disks 0, 1 & 2 are in the chassis.
5.) Disks 0 & 1 run off the mobo's controllers.
6.) Disk 2 runs off a SIIG Ultra ATA/133 PCI controller.
7.) Drives R:, S:, T: & U: are card reader unit (Koutech F7210).

I can disable drives R:-U:, by "stopping" the USB device, although I
don't actually "remove" anything. I was able to get my system to
"see" the missing drive 2 (L, by turning the cartridge key to "off",
then back to "on". ( I don't believe it was the "scan for hardware
changes" that worked. )

So...I'm thinking this has to do with the number of physical "disks"
in the system. ( So far I haven't been able to find anything about
what any such limit might be. ) But maybe it's a matter of the number
of "drives". Maybe some combination. Whatever it is, I would like to
know. I'm also wondering if it would be possible to script the
disabling of drives R:-U: and enabling of drive L:. And...if so, how
to do it. Can I restrict the enabling of drives R: thru U: to perhaps
just ONE (1) drive? ( I only have one Compact Flash card. )

Can anyone out there help me? Any help would be appreciated.

Thanks in advance,

Tom