Having had great success in tuning my XP media server per separate thread, I want to clone the HDD that runs XP and Program Files. All other data is stored in various partitions on physically separate HDDs so this is easy to do using my copy of Acronis True Image.
The most straightforward approach appears to be to buy a SATA HDD of at least enough capacity (I will in fact buy an identical Seagate Barracuda since I have excellent experience with them), and then to mount it in a USB or eSATA enclosure for cloning. Removed from the enclosure, it could then be swapped with its parent without further ado if I ever I needed to do so.
I have never however purchased a "reusable" external HDD enclosure before though, so whilst the job they do appears to be simple and therefore they will all be much of a muchness, this may not be the case.
The one I currently have in mind is
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TVM for advice shared.