Thank you for this very useful information. Reporting on measurements :
Bytes Per Physical Sector = 512 for both partitions (so, what is the conclusion, here ?).
Start Sector number : divisible by 8 for the original partition, not divisible by 8 for the restored partition.
This confirms that the restored partition received the XP alignment.
So I made a new restore, and this time, I was able to set the Windows 7 alignment.
The things I did differently are :
I formatted the small unallocated partition at the beginning of the target disk. It now has a drive letter.
I deleted the existing partition on the target disk, before dragging the saved partition between partition n°1 and partition n°3.
The first time, I dragged the saved partition over the existing one, thinking that it would be overwritten.
Since I got no error message, I assumed this was the natural thing to do.
It's at that point that I was offered only two options : either do this, and accept the XP alignment, or force the Windows 7 alignment, and see partition n°1 (unallocated) disappear.
Looking back, I realize how confusing and unconventional Macrium's interface is. There are actionable elements all over the place, and you never know which one to use.
I image everyday, but I very rarely restore.
The reason I made the 6 GB dummy partition at the beginning of the disk, is to prevent access to a recurring bad sector.
I assumed (maybe wrongly) that this was the reason why Macrium would regularly fail while doing full images, while incrementals would still be (usually) possible.