I think it is quite obvious we are dealing with a number of separate issues. Some are hardware related, others are driver related, and some may even be card related.
I mostly use a old HP NW9440 laptop which, under XP SP3, would read some cards (even 16 GB SDHC ones) but not others, even some 2GB cards. These cards are also not accessible on my old Dell 700m laptop with XP SP3. I just learnt to use an external USB device
that could access the cards. It was all quite random - some hardware devices would read some of the "difficult" cards, others wouldn't.
However, when I upgraded to Windows 7 (skipped Vista), the computer would pretty much read any card I would throw at the NW9440's SD slot. Obviously, *in my case*, it wasn't a hardware problem (the NW9440 has a PCIxx12 TI chip) since they worked under Windows
7 quite well.
Since some of the programs I run that need access to proprietary hardware-formatted SD cards only run on XP/Vista, I decided to revisit the issue. After much hunting and trying out several drivers, I found one (version 2.0.0.8A) on the HP site that appears
to solve the problem. I am now able to use a 16GB SDHC card with XP SP3 as well as the other SD cards that never worked before. This is not the driver that HP has on its NW9440 driver page, of course.
FWIW - if anyone has a HP computer with the same problem, it might be worth giving that HP driver a try. The one that worked for me was 2.0.0.8A, which is SP38607.exe. I have not as yet found a similar solution for the Dell, which uses a PCIxx20 TI chip
and at the moment has driver version 1.0.2.5. I have not installed Windows 7 on the Dell either, so don't know if it would work like the HP did.