some flash drives can take up to 2 seconds to write a single block.
Also, reverting the previous two commits because they didn't actually
help. Aughhhhhh...
So there's a host controller hardware bug on the STM32F407 that
manifests when writing to some FAT filesystems. Transactions will stop
partway through a 512-byte write, and nothing can get them started
again. We can avoid this by soft-resetting the AHB-interface state
machines on every channel halt.
Robert Fisk