NY wrote:

My Windows 7 PC has a front panel with various card readers (SD, CF, etc) in
a module that is connected to a USB header on the motherboard.

Usually the SD reader recognises that I have inserted a card and adds it to
the list of known drives in Windows Explorer. But sometimes it fails to do
so (it's as if I've not inserted a card), and the only remedy is to reboot
or logout/logon.

I'm not sure whether this also applies to the CF drive because I don't use
those cards.

It's not poor contacts because I've tried removing and reinserting the card,
wiggling it, pressing it up/down in the slot. No change. Once I've removed
the card and later reinsert it, Windows simply does not recognise that it's
there. And that is for a variety of cards. I've even tried swapping the card
reader module with one from another PC, and still the fault persists.

The SD-only reader in my laptop (which is also Windows 7) never experiences
this fault, so it's not a generic Windows 7 thing.

When inserted, the USB device is supposed to send its presentation data
to the OS to identify itself by an ID along with type of device. Some
devices don't handle the USB handshaking reliably. Other than get a new
device that works correctly, you could erase the USB device's
enumeration data from the registry. This isn't for the faint of heart
and multi-layered permissions are often involved: the permissions won't
propagate into the lower folders of a registry item so you have to do
them one at a time, and when you grant yourself ownership and
permissions on a folder then suddenly more subfolder show up that you
also have to change their ownership and permissions. There are tools to
help with this but I rarely have to eradicate USB enumerations so I just
spend a couple hours doing it myself.

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\U SB
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\U SBSTOR

There are where you usually find the USB enumeration data. The latter
is where mass storage device types are listed. One tool that I have
used to erase old and sometime corrupted USB enumeration data is
Nirsoft's USB DeView. That often works to list the USB devices and let
me right-click on one to uninstall it (remove its USB enumeration data).

The above is for when the presentation data sent by the USB device is
insufficient, corrupt, or invalid and you need to get rid of the old
data from the Windows registry, plug in the device, and hopes it works
to identify itself again. It is also possible your USB card reader
isn't sending a USB new device notification to the OS. The OS doesn't
see there is a USB device newly attached because the card reader didn't
notify the OS. That's part of the USB hardware protocol and I've not
bothered to delve into it beyond what was necessary to solve my own
problems. In your case, rebooting the computer results in sending a
reset to all components in the computer to set them to a known state.
Either the power cycling (if you are powering down and back up instead
of just rebooting) or the reset gets the card reader working again.

Is the card reader attached to a USB port on the mobo or to a USB port
on a daughtercard? USB daughtercard can have external ports and an
internal port. However, often the internal port is shared with one of
the external ports. To use the internal port means you do NOT use its
paired external port. We don't know what hardware you actually have.

Security software can also get in the way of USB new-device
notifications. For example, some anti-virus programs will alert when a
new USB drive is attached and require the user to accept the new device
via prompt. Well, if the anti-virus program can intercept and interfere
with USB notification then it also cause unwanted side effects. Have
you tried permanently disabling (not uninstalling) your AV software to
check if the card reader starts working again?