kernel panic with greater that 8 GB of memory

[Scott Long]

 > I'm not convinced that the bounce-buffer scheme works on amd64 at all.
 > I don't think twe, ata or bge work (individually or in combination)
 > with more than 4GB of RAM.

Bounce buffers with >4GB were indeed broken in 5.3-RELEASE.  I fixed 
them about a week ago in 5-STABLE and 6-CURRENT.  You can work around
the breakage in 5.3-R by setting the following in /boot/loader.conf:

vm.old_contigmalloc=1

Scott