SD card socket with MMC and Atmel

I don't know whether particular SD cards expose wear-leveling information, but for the most part I would suggest that your desire to avoid erasing blocks that hold FF's is misplaced. Even if a virtual disk block happens to only contain FF's, it will almost certainly contain other addressing information and error-correction data which will have to be rewritten if any changes are made to the block, regardless of its previous content.

I believe SD card manufacturers are free to select their own algorithms for deciding when to rewrite blocks which haven't been accessed for awhile, and for ensuring data integrity in the event of power failure. Consequently, I don't know of any particular method of ensuring that an SD card won't get corrupted if the power fails during a write.

I work for a company that used to be a member of the SD association, we are familiar with the 2.0 (SDHC) spec. The SD card spec has NO entry for wear leveling. That is completely dependent on the SD manufacturer to handle that if they so choose. We have seen that some likely do, while others very much do not (beware the super cheap knock-off SD cards). SDXC may have changed that to include wear leveling, but I am unsure of that. Unfortunately the only way to really show that is to get your hands on the official spec. You can find it online most likely, but the SD association really wants you to pay for it.

As a side note, taking a 2GB card and writing it beginning to end over and over again averages about 10TB before the card is dead and no longer is writable. Also, SD cards will not let you know when data is bad, i.e. wont return an I/O error like a PC harddrive will. This might not be an issue for embedded designs as 10TB is a LOT of data, but it could be a factor for someone.