One of those moments

When attempting to make a slave disk into a master disk (and remove the previous master), it always helps to look at the documentation for the correct manufacturer and model of disk.

There I was attempting to make a Maxtor disk into a master whilst looking at the jumper settings for a Western Digital Caviar drive. Doh.
That’s what I get for trying to change the settings without removing the drive from the case and forgetting which drive was which.