I am using an old Nikon D70 with a 4 GB memory card. It has been working well, with no major errors for over a year now. Today I took ~100 photos, and when I tried to upload them, it only displayed 9 images. However, when I click on the properties of the card, it says there are 3.5 GB of photos, which is about right. I don't usually delete old photos. So where are the rest of the photos? They're on the card somewhere. I checked to see if somehow they were all changed to the "hidden" setting, but they were not. Any thoughts?
There are plenty of reasons why you would only see some images and we cannot tell you why without more information.
However, you can simply get around those by accessing the card directly from the operating system. That means Windows Explorer (Not to be confused with Internet Explorer) on Windows. For Linux, Dolphin on KDE and Nautilus on Gnome or simply any command shell.
Apparently it is called Finder on a Mac (If anyone knows better please correct this).
Just open the card (on Windows it will have a drive letter, say G:) with the file manager and you will see a directory called DCIM, your photos are below there. Under that one you will see one or more directories which have the images. They should ALL be there and you can use the file manager to copy them yourself.
Your program to show the images may simply be showing you only a few because it only looks at one directory (you will have more than one if you used multiple cameras or took more than a certain number of images, somewhere between 500 and 9999, depending on the camera) or it may incorrectly (or correctly) assume that the other images were already copies so it does not show them to you as a feature.
Try formatting the card (either using your camera menu or a computer).
WARNING (just in case) : formatting will destroy all existing data in the card. make sure you have a copy of everything you want from it first.
What has most likely happened is that you have either taken some photos using a different camera, or put some other files on the card which are not visible by your camera (or the software you are using to view/import the photos).