The symptoms was "
uvcvideo: Failed to resubmit video URB (-27)" popping up in the syslog and motion hanging "forever" in diskwait.
Some searching for the error messages suggested that it is a USB-hardware bug which more recent kernels than the default 3.2 in wheezy handles better.
So I installed 3.9 from wheezy-backports.
And then I had to add "
rootdelay=10" to /etc/default/grub, because otherwise the mounting of the filesystem would occur before the assembly of the software RAID!
Took me quite a while to get that in, because for some reason the kernel does some graphics thing that just made the monitor flicker through different colours instead of showing the text console.
Sometimes computers really suck.