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SD card cannot be read in EOS Rebel T7


I have a Canon Rebel T7, a 256mb SD card (full size not micro,) and a Mac.

When I use an adapter and plug the card in it says, “this disk inserted was not readable by this computer.” If I reformat it on my Mac then my camera says that it can’t read the card and I have to format the card on my camera. If I format the card on my camera then my Mac gives the same not readable message. I’ve done so much googling I can’t find an answer!

I’ve tried two different brands of 256mb SD cards. The only card that works on both my camera and Mac is an old 4mb card I’ve had for years.


Is the card size too big? Do new SD cards require a different card reader? (My reader is years and years old. I had been using a Mark II for 14 years and it just died and I’m unable to afford to upgrade.)

It’s been a giant headache as I photograph a service every Sunday and I now have to transfer my photos onto my mom’s PC to a flash drive to get them onto my Mac to edit. She’s only visiting so once she leaves I can’t access my photos. Please help me before I pull my hair out!


Sorry, I definitely meant 256GB. It was after 1am and I was tired and frustrated. After 14 years my Mark II died so this card issue has thrown me off.

I’m wondering if it’s a card reader issue. The card that works in both my camera and Mac is an old 4GB SDHC and I looked at my card reader (it’s 10+ years old) and it only says SD on it. My new cards are SDXC. I’m thinking maybe it can’t read the new cards. My mom’s PC can and my photos are there so it has to be an issue with the reader or Mac.

My Mac is a few years old. It’s running Mojave version 10.14.6. I tried reformatting the card on it to FAT and something else that had FAT in it but both times the camera says it can’t read the card even though I thought I read that it could do FAT.

I’m going to buy a new card reader that works with SDXC and hope it works. If it doesn’t then I guess I’ll try a smaller GB card. Really trying to avoid having to get a PC just for this because I can’t afford it right now.

If you're using SDXC cards with your camera, the format should be exFAT and not FAT.  You may already know this, but you can use the built-in Disk Utility application on your Mac to format SD cards to exFAT.

It does sound like your older card reader is having issues with newer cards.


Camera: EOS 5D IV, EF 50mm f/1.2L, EF 135mm f/2L
Lighting: Profoto Lights & Modifiers

I tried formatting the card on the Mac for FAT and I think it was also exFAT but after I tried both the camera couldn’t read the card and I’d have to format it on the camera in order to use it again.

I’m hoping it might be a card reader issue as my card reader is 10+ years old and it only says SD on it. The 4GB SDHC works just fine in both the camera and card reader so I’m wondering if it can’t read the SDXC ones. I know the card works fine in the camera because I was able to get my photos off it via my mom’s PC.

Makes my head hurt. I’m used to CF cards hah.

"mb" is MEGAbits, not millibits. No such thing as millibits (which would be 1/1000th of a bit if there was such a thing).

I'm afraid that's wrong.  A lower-case "m" is milli.  Uppercase "M" is Mega.  These prefixes don't apply to just bits.  See:

Plus, fractional bits make sense if you're talking about aggregate rates.  For example, video can easily be compressed to 1/3 of a bit per pixel (i.e. 330 mb per pixel).


Page 5 of theT7 manual states the SDXC will work along with two other cards.

Canon EOS T7; EF-S 18-55mm IS; EF 28-135mm IS; EF 75-300mm; Sigma 150-600mm DG

@Tintype_18 wrote:

Page 5 of theT7 manual states the SDXC will work along with two other cards.

Maybe they have corrected the SDXC compatibility issue with xxxxD bodies with the T7.  I do not think they have because there have been several posts like this one in these forums.

Do 64GB SD cards work in your T7?  I tried at (4) different cards and they all eventually failed, or failed to format in the camera.  Formatting in a computer only led to the inevitable failure to read in the camera.  They would work for a SHORT period of time and fail.

If I put the same cards in my 6D or any other midrange body, they would work flawlessly.  Since you seem to be using a T7, then I strongly caution against SDXC cards.  I know what the manuals say.  

The problem does not appear in the xxxD bodies like a T5i, T6i, T7i, etc.

"The right mouse button is your friend."

Not applicable

Just to summarize what is being said here, it appears that the OP's memory card is working fine in the camera, as his mom's computer can see the photos on it. So, it is not a matter of the camera being able to support the 256GB card. His mac computer cannot read the card is the issue. It would seem that his 10-year-old card reader cannot support the large memory card's format. He should buy a new card reader and just to be sure, a smaller memory card (32GB or smaller).

The title of the thread is confusing. He also says the card cannot be read by the camera. I dunno 

"The right mouse button is your friend."