Moving this discussion to the Adobe Digital Editions forum.
There are a couple of possibilities. I'm going to make an assumption that,
when you say you've tried to transfer an ebook to an SD card, you're asking
Digital Editions to do that. If this is the case, ADE does not support SD
cards as a 'transfer device': it's not a supported ereader. You'd have to
use your computer's utility programs to copy the ebook to the SD card from
the ADE library.
The other possibility is that the ebook has digital rights that do not
permit the ebook to be copied. You can check them by looking at the
ebook's Item Info. You get there from ADE's Library view by clicking on
the small arrow next to the ebook title. A drop-down box will appear, and
you can select 'Item Info' from the list in the box. The digital rights
assigned by the publisher or distributor will indicate whether you can copy
the ebook or not.
Hope this helps!