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All About Memory Cards

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.


TypeCapacityTypical FormattingYear of Introduction
SD up to 2GbFAT16 or FAT322000
SDHC2Gb to 32GbFAT322006
SDXC32Gb to 2TbexFAT2009


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:

ClassMinimum Sustained Speed
22 MegaBytes/Second
44 MegaBytes/Second
66 MegaBytes/Second
1010 MegaBytes/Second


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.


SD Card Characteristics

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:

  • File System: FAT32 for SD and SDHC; exFAT for SDXC cards (while Fat16 can be used for SD, use Fat32).
  • Allocation Init Size: 32Kb.

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.


Recovering lost data

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.


  • If you have a corrupt card, deleted files, or formatted the card, immediately cease using it.
  • If you can access the card, try ChkDsk first (or fsck in Linux or run the disk utility on a Mac).
  • Attempt to recover the lost photos using a recovery software.

  • If you cannot access the card with the recovery software, reformat the card in Windows using Quick format (Warning: Do NOT uncheck the QUICK format box), then attempt to recover the files again.


Chkdsk tutorial


SD card recovery tutorial using Recuva


Avoiding File Corruption

The best way to prevent a corrupted card is to do the following:

  • Always turn the camera off before inserting or removing a SD card.
  • Always close any open photos that are accessing your card prior to removal.
  • Always properly unmount your sd card from the computer. In Windows, click on the "Safely Remove Hardware" icon in the Taskbar Notification Area. For a Mac or Linux, right click the SD card and select "Eject".

  • Always replace or recharge your camera's batteries before they are completely dead. If a battery dies in the middle of taking a photo - the card may become corrupted.

  • While it is not necessary to do it after each use, periodically reformat your card in your camera (or computer). There is nothing wrong with reformatting the card after each use, and it will not significantly reduce the card's life. However, it's simply not necessary.

Properly dismounting memory cards tutorial


Accessing the contents of the memory card

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


Trash and Recycle Bins

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.


Best Practices
  • Do not use your SD card for long term storage. Rather, download the files to your computer as soon as you can after your photo session. And make sure you have at least one other backup, such as a USB hard drive. You should always have at least two copies of the photo before deleting them from the SD card.
  • Before purchasing a SD card greater than 16GB, make sure your camera can read it. Older cameras may not be able to read a higher capacity card. Same goes for the fast UHS cards.

  • Consider purchasing many smaller cards rather than a single large card. First, it gives you redundancy should a card fail - you may lose some, but not all of your photos. Secondly, the cost per Gb of storage is often lower with smaller cards.

  • If you shoot video, buy at least a Class 6 card (or class 10 if your owner's manual recommends it). For photos, unless you are shooting RAW format and high-speed continuous shutter, Class 4 cards should suffice. But again, consult your owner's manual. If you shoot RAW/Continuous, consider buying one or two Class 10 cards. No need for all of your cards to be Class 10 as that becomes expensive.

  • Always close out any applications accessing your SD card, then properly eject your SD card from your computer after use.

  • Always wait for the card to finish writing, then turn off your camera before removing an SD card.

  • Always use the brand and model of SD card recommended by your camera manufacturer. Most manufacturers test select brand cards, and while most SD cards will work, there can be some incompatibilities in the different brands.

  • If you print photos from a digital card at your local WalMart or other superstore, offload them to a dedicated SD card. Mark that card as "special"; use anything you wish - Sharpie, paint can, etc. so that you never - ever put that card into your PC. There is a particular "Trojan" called "DCIM.exe" that can affect SD cards used for photography, and what better way to infect your system at home than sticking the card into a public computer at the photo print store. When you finish printing your photos, reformat the card in your camera before putting it into your PC. Unlike computers, cameras are fairly immune to viruses (at least for now).


SD Class and Speed Ratings
SD Card Types
SD Data Recovery