Agree with this. Thats why I would make a mirror and then break it. so that the c is backed up and bootable
For under £400 you could buy a pair of new 300gb 15k SAS drives and migrate your entire file systems onto them, keeping the original drives in tact.
Dont forget that you can make symbolic links in NTFS so D:\foo appears under C:\bar.
A mirror as a backup? Right..
If it's a mirror of the boot disk you'd need to put the MBR on the mirror clone too of course.
The minute you allow yourself to think, "ah, should be ten mins to resize the system volume..." is when you can kiss goodbye to your evening