Problems with RAW files transferred via SDHC

Hi,

Hoping someone here can help me out with a strange issue i'm having. I recently got a new Canon 450D and i'm having some weird issues with transferring the RAW files to my iMac (I previously had a 350D which used CF cards and never had an issue).

If I try and view or download the RAW images via a number of different SDHC card readers they come out looking like this.

(example taken from Adobe bridge but is the same with Lightroom, Aperture, Preview etc. . .)

However, if I download the images directly from the camera via USB they are all perfectly fine when viewed in any application. I have tried a number of different cards, readers and USB ports and the result is always the same. Even stranger is if I boot the iMac into Windows 7 via Bootcamp and view or download the images via SDHC they are all perfectly fine??

Given that the images are ok on the mac when downloaded from the camera i'm pretty sure this is something to do with the SDHC, but I just can't figure out what's causing it.

I have searched pretty much every combination of keywords for this problem but there doesn't seem to be an answer anywhere? The only thing that comes close is corrupted RAW files due to hardware problems/failure but I have ruled this out due to them being ok when transferred from camera.

Any help would be greatly appreciated as I just don't know why it's happening?

Thanks