I have wanted to store various items on a disk or CF card, logs, graphs, userman data, dude data, etc..
Currently the only way usage of an alternate storage disk (other than your boot disk) is web-proxy cache storage. As far as I know that is the only feature in RouterOS that can use any other disk.
As for your question about queues, queue graphs are stored on the disk but the queues them selves 'store'/buffer the queued packets in RAM not on any disk. You would not want to store queued data on a disk, the latency would be horrible.
When using the disk/CF card for proxy, it will format and prepare it for usage. FAT-32 formatting is un-needed, I believe it formats it as ext2 or ext3 anyway.
If anyone plans to install a new userman system, I recommend installing the RouterOS for it on a new Hard Disk in a x86 machine instead of a flash disk in a x86 PC or a routerboard. The userman logs and database can quickly exhaust the 64MB flash in most routerboards.
When Mikrotik released the new rb433AH that included a microSD slot, there was mention of a new 'feature' that would be announced shortly that could use it. Newsletter 10