When Apple announced the swapping of the ExpressCard slot on the 15-inch MacBook Pro for an SD (Secure Digital) memory card slot, the few users of ExpressCard-compatible peripherals - at least those other than SD card readers - were understandably disappointed. (Apple claimed that only a "single-digit" percentage of MacBook Pro users used the ExpressCard slot.) For most people, the addition of the SD slot is welcome, since the majority of consumer-level digital cameras use SD cards for storage. Nevertheless, it didn't seem like that big of a deal either way.
However, a recent Apple KnowledgeBase article reveals an extremely useful and previously unmentioned feature of the SD card slot: users can boot the Mac from an SD card with Mac OS X installed on it.
To make a bootable SD card, you must first change the default partition table to GUID using Disk Utility, and format the card to use the Mac OS Extended file format (as opposed to the FAT32 file format). You can then install Mac OS X onto the device, enabling it to boot the Mac, which could be very handy in a troubleshooting situation.
The MacBook Pro SD card slot accepts cards that conform to the SD 1.x and 2.x standards. This includes Standard SD cards, which hold between 4 MB and 4 GB; SDHC cards, which hold between 4 GB and 32 GB; and the older MMC cards. MiniSD, MicroSD, MiniSDHC and MicroSDHC cards can work if used with adapters that enable the cards to conform to the necessary physical configuration. While the MacBook Pro can read (but not boot from) cards that use the FAT32 file format (the standard for most SD cards), cards that use the exFAT system will not work.