I am running Windows 7 Pro 32 bit and have my system and files backed up and mirrored to my external hard drive.
If I upgrade to the Windowss Pro 64 bit disc can I then use my external hard drive to put all my programes and files back onto the 64 bit system?
A hard drive is not 32bit or 64bit dependant. You OS (e.g Windows) is. So, if you format and reinstall Windows you will lose everything that is present in the drive or partition that you delete. You will have to reinstall programs, too.
So, moving or copying simple files (e.g docs, photos, music, movies, but not programs) to external drive is definitely a good idea, and you will have no problem to access them with a 32 bit or 64 bit Windows. Some few programs may still work even if you don't reinstall them, but most of them will need to be installed again.
so its not a simple case of, after formatting my pc and putting the 64 bit version of Windows 7, then just plugging my external in and running the restore program
No its not that simple. On the 32 bit version of Windows 7, programs get installed to the "Program Files" directory. On the 64 bit version, 32 bit programs get installed to "Program Files (x86)" folder and 64 bit programs to "Program Files" folder. I'd reinstall your programs the normal way.
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