I've booted into 1.4.2 off of an external hard drive with my 
PowerBook 145B and my wife's IIci without difficulty.  I acquired a 
Quadra 700 with 20 MB RAM, attached the same external drive to the 
SCSI port, and have tried to boot several times without success.  The 
booter goes through its routine, the boot process starts, the screen 
goes black and the process stops cold.  I am using a boot disk with 
System 7.5.3 (7.1 won't boot the computer) and the Booter renamed as 
Finder.  The Quadra's internal hard drive has OS 8.1 installed and 
the external drive, with 2GBs of NetBSD partitions positioned at the 
start of the drive followed by a 2GB  Mac OS partition, has  Mac OS 
7.6.1.  I can't think that there would be any problem with the files 
on the Mac partitions since the boot process doesn't even get into 
probing the hardware.

FWIW the internal drive is sd0 and the external drive is sd1 and is 
properly specified in the Booter (like I said, this same drive worked 
fine for booting into NetBSD on two other computers).  But the kernel 
doesn't even get started on the boot process.

The log reads:

Logging to bootlog2
Booting...transparent = 1
MID_M68K executable: entry 0x2e00
1392708+74448+176500+177785
Attempting to kill all running programs...
Changing monitor settings...
   Bye-bye...
      So I sez to him... the real way
      that it should be done is to...

The screen goes black and the boot process dies without further ado.

I couldn't find anything like it in the FAQ, and the Q700 is supposed 
to be a supported machine.  Any suggestions?  Thanks!