Booting from a USB disk

[Vincent DEFERT <20.100%defert.com@localhost>]

Hi,

I've installed NetBSD on an external USB disk and made sure its 
/etc/fstab was correct.
However, when I reboot on this disk, I have a "Heap full" error and the 
machine boots on its internal disk.
If I drop to the boot prompt and type "boot hd0:", NetBSD boots normally.
My guess is that I need to set some additional option in some 
configuration file.
With FreeBSD, I need to set vfs.mountroot.timeout="10" in 
/boot/loader.conf, for instance.

Can someone tell me what I should do for NetBSD in this case?

Thanks in advance,

Vincent