SD cards were originally only designed for capacities up to 1GB. Changes were made to the specification which allowed capacities of 2GB to be realized. But, in order for a device to be able to use a 2GB SD card, the device must be designed to address the memory space. Unfortunately, not every device is able to do this, and there is no easy way to tell if the device is compatible without first calling the manufacturer of the device.
If a 2GB card is put into a device that only supports 1GB, the host device may ask to format the card and repartition it to 1GB. This applies to full size SD, miniSD and microSD cards.
So, for example, I insert a 2GB SD card into a built-in card reader on a computer that is 4 years old. When I attempt to access it, a need to format message may appear, and if I were to format the card and then check the capacity, it will read somewhere around 980 MB, instead of the 1.9 GB that I was expecting.
This will happen with any host device that is not compatible with 2GB cards; it can be a mobile phone, built in card reader, GPS unit, etc.
It is always best to contact the manufacturer of your device to find out if it is compatible with a specific capacity SD card.
If a card has been repartitioned to 1GB, it may be “fixed” by using a 3rd Party repartition tool. Windows can not repartition USB Removable Disks in Disk Management. A good program to use is the free HP USB Disk Storage Format Tool. If you do an internet search for this tool you will find places to download it.