I know that Secure Digital cards are capable of having a password set to control access to the data on the card, but I don't see any way of setting this on my Mac. Maybe because I'm running an old system 10.5.8, or am I looking in the wrong place?

See this CultOfMac article for using FileVault to encrypt data on any drive including thumb drives and SD cards. It was the first hit on a Google search.

yes SD has a hardware level lock on it that you can, in theory, lock the data in such a way as to not have access to it (in any form) unless you provide the sd card itself with the key.

Although this "feature" was a bit concerning when it came out, it seems to have rarely gotten used. Maybe by some government types. I don't even know if Mac OS X or any other OS such as WIndows has the capability to access a secured SD card. I've never encountered one nor heard of anyone else that has.

Although I DO find some other interesting points. First of which is that my PowerShot S3IS camera can boot off alternate firmware to gain access to additional features. This boot process, curiously enough, requires the SD card's "Locked" slider to be in the LOCKED position to boot. And while booted off said card, pictures can be taken and stored normally. (this Lock slider is unrelated to being "Locked" / "Secured" by the Secure Digital card's namesake password protection)

Naturally that gets types such as myself curious, and I took apart a 32mb (thanks canon for that bundled card!) to see about the lock, and as expected, it's just a plastic slide that has no electrical connection. The locking function is entirely regulated by the device you plug it into. (I've encountered people on more than one occasion that insisted that the card could not be accidentally erased if it was in the Locked position) Reminds me of client-side security. And of the Write Protect notch/slider on 5.25" and 3.5" discs.

Makes me wonder what sort of security it really has when "Secured". I bet it's just another doorman like the Lock tab, and that your data isn't encrypted in the least, it's just a little harder to get at.

My understanding is that the data is not encrypted. The 'security' comes from the fact that the SD card has a built in processor that acts as interface between the host computer, and the data. So, in theory, it would be necessary to break into the chip itself to retrieve the data without the password. I think this is likely adequate to protect the data against anyone who doesn't have a lot of very expensive equipment available, and certainly good enough to prevent casual inspection of data files. But for items of national security, maybe not.

Filevault is probably the way to go then for data that will only be read by my Mac.

It was more out of curiosity that I asked about this. But, upon further consideration, it's not really practical, since there don't appear to be any cameras or audio players that can deal with the security either. So, there wouldn't be any way to move password protected data to or from these devices.

"file vault" is probably not a good way to describe it. File Vault is using an encrypted disk image for your home folder. What's needed here is merely an encrypted disk image.

I use a hybrid system here. I have a 10mb encrypted disk image that has my password list, software license codes, ssh keys, and some other things on it. It's "primary" location is on a usb flash drive I carry with me at all times. (64gb, bootable) When I make changes to it, the (locked) copy on my computer's hard drive gets updated. It's locked so I'm not tempted to make changes there instead of on the flash drive, which may later get overwritten.

I have a cron job that makes sure the dmg stays mounted. It can script the remounting without providing the password because the password is in my keychain. In this way I am able to keep my ssh keys there also, and automated jobs like rsync network backup via ssh key can run automatically. But if my computer is ever stolen, they won't have access to places I have shell access to by ssh public key because they cannot mount the disk image without my keychain being unlocked, which won't happen if they reset my login password. Same goes for my usb flash drive, there's no risks if it's lost.

Just realized that you can't use Filevault on individual volumes. It only encrypts the home folder.

Virtual1, I wasn't able to follow how you're doing it. You lost me with the encrypted disk image.

I may just look around for some separate encryption software. I haven't used it in a long time, but I assume PGP is still available somewhere.

I don't have a pressing need at the moment, but the convenience and portability of SC cards puts them at risk of being lost. So, if I do use any for personal info, I wouldn't want to carry them around unless they were protected.

BTW, My interest in this was piqued after I bought a small music player that takes SD cards, and I got wondering why they were called "Secure Digital." So I had to look it up on Wikipedia.

It's simpler than you may think. See How to create a password-protected (encrypted) disk image for the procedure. If you'd like still more info, Google 'encrypted disk image' and brace yourself...

Thanks for the link. That's easy to follow, even for me.

However, there seems to be a glitch. I tried this with a 32GB SD card. I decided to create a disk image the same size as the SD card. Disk Utility then informs me that it has to reduce the size to 28GB, as that is the actual space available. Okay fine. Then it comes up with an error message saying "File size too big." Okay, so I keep trying smaller and smaller sizes until I get down to about 100MB, and then it works. Sort of. It seems to have created the disk image. But it's now giving me a message saying "Erasing Free Space. Estimated time remaining: 6 hours." This seems a tad inconvenient. Have I done something wrong?

It's not necessary to have DU erase the free space.

Generally speaking, you'll find it faster and easier to create the disk image on your hard drive, then move it to your sd card, rather than creating it on the sd card.

I have no idea why it started erasing free space. Maybe I accidentally clicked something I shouldn't have.

it may require double the space to make the disk image. In that case, make the dmg on your hard drive, say 27gb. then once it's created, drag and drop it to the flash drive.

In retrospect, it probably makes no sense to have the dmg the same size as the SD card.

Question: Is a dmg compressed?
(I assume it is. Otherwise, why would it be the method of choice for distributing software online.)

Question 2: What is a sparse disk image, and what is a sparse bundle disk image?

A .dmg MAY or may not be compressed. It may also be encrypted if desired.


A sparse disk image is a form of disk image file that can grow or shrink depending on the size of the files contained within the image.

A sparse disk bundle is a directory containing a number of files or bands, each a fixed size (8.4MB in OS X 10.8) that makes it possible for Time Machine to more efficiently manage its data.

To elaborate a bit on Joemikeb's response, a sparse bundle image looks like a single file, but is in reality a directory containing separate files. The format is used for backup purposes like Time Machine. A sparse bundle makes updating easier and faster, because only changed items can be updated instead of forcing the entire image to update. See sparse image for some details, and this discussion for some implications for the end user.

I'm posting here rather than starting a new thread, because we're on the subject, and the answer may be important to Bob_00001.

As you can see here, there appears to be 6Mb of overhead in my sparse image; is that expected, is it unusually large, or am I failing to understand what I'm seeing?

Thanks.

yes there will be a directory of what blocks are unused. it's a bit like a free space map in a volume. it indicates which blocks are unused (and thus not taking up space) as well as serving as an index as to where in the sparse image to find each used block. (now called "bands" I think)

Thanks for that; it's pretty much what I thought, but 6Mb seemed a bit large, so I thought I'd ask.

> (now called "bands" I think)

As per joemike in post #24967, the term "band" applies to sparse disk bundles.