The subject of problems with USB flash disks has ben raised here in
the past.  Briefly, for many people (myself included) many USB flash
disks (I have a M-Systems DiskOnKey) don't work a lot of the time.

Last time this came up, someone suggested that I should have a USB disk
inserted at boot time.  This works, 100% of the time, on 1.6.x and
-current.  If I don't have a disk inserted at boot time, it fails
very often -- possibly 100% of the time, but I haven't checked.

The failure symptom is that it never progresses past saying
"umass0 at ....".  If you remove the flash disk, the system dies.

This sounds a lot like an initialization problem that should be
fixable.  Is this enough information for someone who understands the
USB subsystem well enough to hack on it?  (I should note that a disk
that's inserted at boot time is often not recognized until it's removed
and reinserted.)