Monitoring SD/TF Card status while interfacing with separate host
I've played quite a bit with MCU and sd cards, but at the end of the day, an embedded Linux still the best solution if you don't want to spend countless hours working on filesystem stacks to make it work on MCUs while linux already has all the support you need.

You can have a cheap small form factor system with the following:

Platform: Atheros AR9931 type of chip with OpenWRT or something equivalent. There is plenty of modules board available that has some IO and the ram/flash for about the same price as a high-end MCU.

To control the SD cards:


Option A is SPI (or specific card protocol) with a multiplexer to access specific cards, you can basically have an unlimited amount of SD cards. 
Option B, easier, use the USB port, with USB hub chip (you can chain several) and card reader chips.