A couple of issues here:
Are we talking a removable hard drive or a external portable drive?
If we are truly talking a removable drive, there will be a hardware driver problem. Unless all the computers he wants to use have the EXACT SAME hardware configuration, at the very least he will have video problems (resolution and color depth). At worst, the drive may refuse to boot.
If we are talking an external portable drive, then it has to have the capability of being bootable. A bootable floppy could get around THIS issue, but you still have the hardware driver problem. A downfall to this solution would be that there is no guarantee that the portable drive would be assigned the same drive letter on every computer it is attached to.I have run into problems in the past where an installed Zip drive was actually assigned the letter C, even though there was also an installed hard drive. There are actually 2 issues here:
1 - The bootable floppy will really only consist of a batch file that essentially says "Go to the attached hard drive and run Windows from there." If the portable drive is not consistently given the same drive letter, the batch file may point to the wrong (or a nonexistent drive).
2 - If your user does simply clone his laptop drive to the portable disk, chances are it won't work anyway. The Registry uses absolute paths (i.e. c:\windows\system32\xxxx, or c:\program files\Microsoft Office\xxxx.exe) when referencing system resources, file locations and application locations). If the cloned drive is installed as any other drive letter, chances are nothing will work and the OS probably won't boot.
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