I am limiting this text to SD, SDHC, and SDXC cards as these are the most popular. Virtually all cameras today use SD memory cards in one of the three versions. SD (Secure Digital) cards can have up to 2Gb of storage, SDHC (Secure Digital, High Capacity) up go 32Gb, and SDXC (Secure Digital Extended Capacity) with a capacity of 64Gb to 2Tb.
Typical SDHC Card
Type: Designates the type of card; SD, SDHC, SDXC. Essentially the difference is the card's capacity. However, the device you are using must recognize that type of card. Unless a camera was manufactured in 2010 or later, it will not likely recognize SDXC cards.
Capacity: Designates the Card's Capacity in GigaBytes.
Class:The Minimum sustained speed. The symbol "C" with a number indicates it's class. The Minimum Sustained Speed is usually important for video as a continuous stream of data must be written to the card. The class ratings are as follows:
Note that UHS-I and UHS-II cards are available (Ultra High Speed) for SDXC and some SDHC types, that can support up to 104 MegaBytes/Sec for the UHS-I, and 312 MegaBytes/Sec for the UHS-II. However, they are still fairly expensive and may only work on certain cameras. As of this writing (Aug 2011), the Nikon D7000 is the only camera I am aware of that can take advantage of UHS-I cards.
Speed: Maximum "burst" write speed. This measures the maximum performance of the card under short - bursts of data, such as when a camera is shooting RAW files. The faster the card, the faster it can accept the next photo.
Write Protect: A sliding tab on the left side of the card that prevents writing, overwriting, or erasing the data on the card. When the tab is up, the card can be written to. When it is down, the card is read-only and cannot be written.
While the Class and Speed may be confusing at first, remember that the Class is the minimum sustained write speed, and is important for streaming data such as Video. The Speed is the maximum short-duration burst write speed, that is typically required by still cameras. So, Class = video performance, Rated Speed = photo performance.
Life Expectancy: The MTBF (Mean Time Between Failures) is one specification that seems to be elusive. However, the memory type typically used for SD cards suggests a lifespan of 100,000 writes. While this means there is a definate life span for SD cards, they should last quite some time in normal use.
Formatting: Cards should be formatted in the camera whenever possible as this prevents incorrectly formatting them. If you do format the cards in your computer, whether it be a Windows, Mac, or Lunix operating system, the formatting must consist of:
Formatting in NTFS, HFS, EXT3, or any file system that might be the default for your computer's operating system will cause the card to be unreadable by the camera.
Myth #1: You should defragment your SD card. While defragmenting a hard drive will improve performance, it is not necessary to do so with a SD card. A SD card, being a solid state device, does not suffer from the mechanical delay associated with having to read fragmented blocks, so there is essentially no degradation in the speed over time. Defragmenting a SD card will not improve it's performance.
Myth #2: You must format your SD card after every use. Reformatting can fix a corrupt card, but it is not necessary to do so after each use. However, you should always turn the camera off before inserting or removing a memory card, as well as dis-mounting it from your computer prior to removal.
Myth #3: You must format your SD card in your camera. While this is generally recommended, it is not technically necessary. However, if you format your card in your computer, you must use the appropriate file system format (FAT32 or exFAT), and block size. While you can use any block size from 4kb to 32kb, cameras almost always use 32Kb as that is the most efficient for large photo and video files. If you don't have a clue about this, formatting in your camera is the best practices.
Myth #4: You cannot use a memory card in multiple cameras. This is quite possible as there are standards that all of the camera manufacturers follow. In a SD card, there will be a folder called DCIM (Digital Camera Images). In a sub-folder under that folder will be a unique folder for each camera. With some cameras, you can change the folder name, but in other cameras you cannot. If you use the same card for multiple cameras, you will find multiple folders under the DCIM folder.
Myth #5: If you delete the photos on your SD card accidentally, they are gone forever. This may be true, but there is some chance of recovery. If you delete the files by mistake, do NOT use that card in a camera until you recover the files. If you continue to use the card, the files will be overwritten.
Sometimes the worst happens and you end up with a corrupted memory card. To recover the files, obtain a file recovery software, such as Piriform's Recuva (recommended) which is free, or SanDisk's RescuePro, which is not expensive. Even then, there is not a 100% chance that you can recover the files. And if you formatted the card, you may or may not be able to recover the photos - it all depends on how the camera formats the SD card.
SD card recovery tutorial using Recuva
The best way to prevent a corrupted card is to do the following:
Properly dismounting memory cards tutorial
There is an orderly hierarchy in the way SD cards are organized when used with a camera. The Japanese Electroincs and Information Technology Industries Association (JEITA) has published a specification; Design rule for Camera File Systems (DCF) which defines a file system for digital cameras.
At the heart of this design specification is the DCIM (Digital Camera IMages) folder, which is the primary location of all photos. Videos may or may not be in the same folder.
In addition, camera manufacturers often add temporary indexing files in the SD card, and there seems to be little standard here - as they can be found scattered all over the place - depending on the manufacturer.
While you can use a single memory card for multiple cameras, memory these days is inexpensive, and many photographers dedicate a single memory card per camera. However, should you share a single memory card with multiple cameras, the Cardinal rule is you must never let one camera access another camera's images, or the memory card could become corrupted. The reason for this is that camera file handling systems are rather primitive, and they do not expect to find anything but the files they created in the memory card.
The DCIM folder prevents this by requiring each camera to have it's own sub-folder.
Accessing memory card contents tutorial
Most computers, whether they are Windows (Recycle Bin) or Macs (Trash), have a safety feature that when you delete a file, it really isn't deleted. This is done so you can recover the files at a later date if you need to. For that reason, the file is actually on the device, so even though "deleted", it still takes up space.
This process is a bit cleaner on Windows machines than Macs as it's Recycle Bin is only active for a hard drive, and disabled for removeable drives such as SD cards. Macs on the other hand store their Trash bin on both hard drives and SD cards.
This is not usually an issue for hard drives since they are so large these days. However, on a SD card, they can cause the card to remain nearly full, even though you believe you deleted the files.
On a Mac, when you delete a file, you select the option "Move to Trash". This does not really delete the file, it just prevents it from being seen. To actually delete the file and recover the space on the SD card, you must also go to "Finder", and then select "Empty Trash", which then deletes the files from the SD card.
In Windows, you don't have to worry about emptying the Recycle Bin as it is not active on SD cards - only on hard drives.