I returned for most of the day to the subject of this thread, and thought I might resurrect the thread itself, having had another go at replacing the hard drive in my 2400c with a CF card.

This time it was a success. As suggested above, the problem last time (apart from the the time-consuming nature of the work on a 2400c!) was evidently the CF-IDE adapter used. This time, I replaced it with another known to work (which had been in a 2300c), and the machine booted right up from a CF card that I had already installed a system on in a PCMCIA card.

The internal adapter used was nothing fancy, just another cheap, generic piece of Chinese origin.

MacBench was run on the machine with the old conventional hard drive (a c.1998 Fujitsu 3GB MHD2032AT) before doing the swap, so when I get a chance I will run MacBench again and see what the benefit (if any) is — beyond the silence. I know that when the machine ran from the CF drive installed in a PCMCIA adapter card, the machine was perceptibly slower than when running from the conventional hard drive, so we'll see if the installation internally makes any difference.