You have several options:
1. 2.5 inch hard drive / SSD via Ultrabay Adapter + Ultrabase. Granted, this isn't exactly "internal",
but it's the easiest and least hackish. Bootable. Speed slightly limited by the Ultrabay's PATA interface, but still likely the fastest among these options.
+ micro SD cards or MR04
+ SD card in second mini PCIe slot, or
+ USB drive in second mini PCIe slot. Both 2.1 and 2.2 uses the USB lanes in the second mini PCIe slot, and appear to the system as USB drives. Note that the second mini PCIe slot only has USB lanes
, so you cannot use any real PCIe cards or mSATA SSDs in it. Might be bootable as USB drives, but you certainly cannot install any Windows systems on it easily.
3. SD card in SD reader. It is sllllllllllllllllllllllllow. Not bootable.
4. CF card + PC card adapter in PC card slot. 16 bit adapters are slllllllllllllllllllllllllllow and only supports small cards. 32 bit adapters are impossible to find. Not bootable.