That will do a verbose boot and show you what devices are being probed.
The usual case is that some pci device is being mis-identified and confused
with another type of device. "dmesg" after the system comes up will allow
you to make a copy of this output to a disk file.
If you can identify the device which is mistakenly probed you can use the
boot -c dialog to disable the devices you don't have. Often disabling the
mis-probed device will cause the true device to be successfully probed
later. Usually its the ethernet card getting mis-probed.
If this works please post your -v output and what you discover to the
comp.unix.bsd.freebsd.misc group. Tell us what kind of motherboard it
is and the specific type of devices you have. The core team may be able
to add some corrections to the 4.8+ and 5.0 branches.
When installing with 5.0-RELEASE, I get an error while booting from the
Fatal trap 9: general protection fault while in kernel mode
ad0: READ command timeout tag=0 serv=0 - resetting
ata0: resetting devices...
When installing 4.5-RELEASE, I get an error after I enter CLI mode:
Probing devices, please wait (this can take awhile)...