If the card is write protected (WP is active), FSBL can not boot and returns the following error:
Xilinx First Stage Boot Loader
Release 2013.4 Feb 4 2014-09:45:56
Devcfg driver initialized
Silicon Version 3.1
Watchdog driver initialized
Boot mode is SD
SD: rc= 0
SD: Unable to open file BOOT.BIN: 1
FSBL Status = 0xA009
The work-around is to route the PS SDIO WP pin out to EMIO and tie it to 0.
This issue has been fixed in release 2014.1.