What was first thought to be only applicable on hard drives is now found on a USB key made by PhotoFast, a Japanese company focusing on flash media adapters of all sorts. The aptly-named JBOD USB Thumbkey accepts up to 4 MicroSD cards, and employs drive spanning protocol (a.k.a. Just a Bunch of Disks) to combine these cards, which can be of different sizes, into one single partition. As it supports MicroSDHC, the maximum capacity is 64GB when all slots are filled with a 16GB card.
The PhotoFast JBOD USB key is also particularly useful in recycling the MicroSD cards we come across from our cellphone purchases. Even if those MicroSD cards are only 1GB to 2GB in size, you can put them together to form a flash drive large enough to transfer files that would otherwise be too big for these cards to hold individually. Available here for $62 USD (ouch)
, the JBOD USB Reader is a green gadget that is quite worth money in the long run.