I want to develop an embedded system for storing incoming streams of
data up to 56 Kbit/s as files on a standard USB pendrive of up to 1
GByte capacity. These files must be named by the system and put in a
filesystem that a Windows PC can read when the pen drive is taken out
of the embedded system and stuck into the PC.

The SBC would not be compressing or transforming the data - that will
most likely be done by a hardware codec - so there is no computation
overhead to consider other than just packing the data into the
filesystem.

My question is: Are there any single-board computers/cores that come
already equipped to do control a pendrive as part of their development
system/documentation/example code?

I have no preferences on architecture or embedded OS, if any. I want
to compartmentalise this one, difficult data capture job inside an SBC
and run the rest of the project's simple logic on an 8051, which is no
problem for me.

Cheapest board - cheapest dev system - smallest board is the order of priority

It's an easy $25 for anyone who already knows the answer, but it would
save me several days reviewing sales litrature.

regards,

Nigel