|If you're trying to read the memory card on a computer.....|
USB devices may not work correctly when they're connected to certain USB ports e.g. ports in a hub or on the front of a desktop case. If you have a desktop computer, you may have an IRQ sharing problem you need to fix, or your power supply may be starting to fail.
See response 3 in this:
Also - if you are using a USB extension cable, some of them have inadequate wiring and will not work properly with some USB devices.
The flash drive (or external drive, or memory card reader) must be plugged into a port it will actually work in.
How to tell if the flash drive (or external drive, or memory card reader) is properly recognized as a hardware device, even if you can't access it's data.
Programs you can try to recover your data, if that's okay.
The most frequent reason people can no longer access the data on a USB flash drive, USB External drive, or a memory card on a computer
See Response 2: