More attempts to install WinXP on an external HDD

From what I had seen, the ngine guide ONLY works if your computer supports booting to an external HDD. Now for years I thought that Dell computers did not!
So, I posted to Dells forums and was surp[rised to find that in fact they do, or at least, nearly all of the Dell desktops we have here do, or at least should. Very interesting thread.
However, my 4550 Dell did not behave as expected when I pushed the F12 key during initial boot in that no usb devices showed up in the alternative menu which showed up.
But, on the E520, both the external HDD and a usb key I plugged did (or at least seemed to).
Then today, I tried to see if the hacked WinXPPro.ISO could be installed to my Seagate usb drive using Margaret's computer (E520).
However, no luck here.
First time, got BSoD when all of the initial files were installed and screen said (on bottom) Setting up Windows.
Tried also with Margaret's 500 GB ext HDD in place of my Seagate but very same result.
Ran a CHKDSK /F as recommended in BSoD, but no change after this.
Tried also with my original WinXP install CD but, once again, got exactly the same result.
Maybe I'll try it on Cormac's machine (9200) as a last resort.