Further to this, I hacked the install initrd to include a sleep (I am not recommending this a solution! :-) ) in cdrom-detect.postinst to test my theory on the usb drive not settling and the behaviour is as I would expect (usb drive detected correctly mounted and installation continues).
Small diff of my /var/lib/dpkg/info/cdrom-detect.postinst attached.
Further to this, I hacked the install initrd to include a sleep (I am not recommending this a solution! :-) ) in cdrom-detect. postinst to test my theory on the usb drive not settling and the behaviour is as I would expect (usb drive detected correctly mounted and installation continues).
Small diff of my /var/lib/ dpkg/info/ cdrom-detect. postinst attached.