USB disk boot issues

[Peter Steele]

We clone systems from specially prepared USB flash sticks and this all works well, except that occasionally the flash stick fails to boot. It fails at the "mount root" step, saying that it cannot mount the specified root partition. We use a labeled partition on the disk to make it device independent, so we have something like

/dev/label/usbroot  /    ufs    rw     1      1

in our fstab. When it fails it doesn't recognize this device, and even ? doesn't list it as one of the available devices. If we reboot, it's likely the problem won't occur again. It seems to be some kind of timing issue. Is this a known problem?