I guess that was the case - I resorted to jurying rigging the disk and DVD-ROM in my main system which had all the other drives disconnected and that one worked fine - though it did have a PS/2 keyboard. After returning them to the original system; booting from a Knoppix thumb-drive to correct the numbering in `/etc/fstab` and temporarily editing the GRUB entry to also compensate for the drive being differently numbered compared to when it was on the installation system I now have a bootable system - hooray! So I have worked around the problem - which seems to be confined to the installer and is a nuisance - but in the scheme of things I guess this issue for me can be relegated to a low priority. For the record the system concerned was a Dell Dimension 5150 with a BIOS version A05. Having read: https://en.wikipedia.org/wiki/USB_human_interface_device_class#Keyboards I think I start to understand why things might go wrong.
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