There are a lot of myths and mis-information about memory cards. Here is the straight scoop:
1. The number 1 reason for memory card corruption is removing the card from the computer without properly dis-mounting it, or removing it from the camera without turning it off.
2. Using a memory card on multiple computers does no harm. .
3. Cards can be formatted on either computer or camera - one method is no better than the other (however, you have to format it properly in the computer).
4. Once you remove the photos from the card, you can put them back - if you do it properly.
5. Unlike a hard drive, it is not necessary to de-fragment a memory card.
6. If you accidentally format the card or delete the files, you have a good chance of recovering them. It is not a 100% chance, but if you don't use the card after deleting or formatting it, chances of recovery are high.
Detailed Information:
1. Corruption: First, the card might be able to be read by your computer rather than your camera, as cameras don't seem to be quite as robust. If you can do so, copy the files off the card then reformat it using a long format (or low-level format in some Canon cameras).
2. Next, you can try to recover your card by using ChkDsk (Windows), or fsck (Mac or Linux). ChkDsk will scan the card and repair any corruption on the card.
3. You can try to recover the files if you formatted the card (using a quick format) or deleted them. This will not usually repair a corrupted card (see #2 above). There are a few free programs, but I use SanDisk RescuePro, which costs a few dollars.
4. When you format a memory card, you can use Windows, Mac, or the camera. The camera will properly format the card, but you have to make sure to set your computer to format using Fat32 and a 32Kb allocation block size. Any other combination and the formatting in the computer will not be as efficient, or even not work.
5. If you want to place the photos back on your card, you must place them in the DCIM (Digital Camera Images) folder, and in the proper sub-folder under DCIM. And you cannot change the filenames or modify the files in any way (i.e. do not do any post-processing in photoshop, etc). The camera is looking for a specific filename format in a specific folder. If the files are not returned to the card properly, the camera will not read them - and in some cases report the memory card unusable.
6. You can safely use a memory card for multiple cameras as each camera writes to it's own subfolder under the DCIM folder.
7. Defragmentation is not necessary as unlike a had-drive, the card is not an electro-mechanical device. There are no heads to move around on a solid state memory card. Memory cards are directly addressable, and it makes no difference where each block of the photo file resides on the card.
8. It is not necessary to reformat a memory card after each use - but it doesn't do any harm to do so.
Best Practices:
1. Always unmount your memory card properly. In Windows, click on the "Safely Remove Hardware" icon in the right-side tray. In a Mac or Linux, right-click on the memory card and select "Unmount". In a camera, make sure you turn it OFF before removing the memory card.
2. Always format a memory card when you first receive it to make sure it is using the proper allocation block size. This allows the card to be the most efficient and fastest. This can be done in either the camera or the computer.
3. Always "long format" the memory card after corruption. Short format is not good enough as it may not fix the card. Many cameras cannot do this, however some Canon cameras have a "low level" format that is essentially equivalent to a long-format in Windows. If you must, long-format in Windows, then reformat in the camera if you don't know how to setup a 32kb allocation block.
4. Always backup your photos in more than one location. Memory cards, like any other device are prone to failure. And never use a memory card for long term storage. I keep no less than 4 copies of every photo on a combination of the computer, network storage, and USB hard drives.
5. Always buy the brand, class, and speed of card that is tested by your camera manufacturer. While you may likely be able to use other brands, avoid cheap cards as they invite problems.
6. Never directly take a memory card to a kiosk (WalMart, etc) for printing photos. Always download them to a USB drive or dedicated memory card for that purpose. Then after printing, immediately format the card. Sometimes kiosks are breeding grounds for computer viruses, and at least one virus (DCIM.exe) specifically attacks camera memory cards.
http://www.althephoto.com