Here are my opinions. Not much different from everyone else.
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Is there a painless way to migrate some programs from C: to D: ?
Unless they are 16 bit programs, the only way to transfer them from C: to D: is by uninstalling from C: and reinstalling to D:. If you move the program folder and try to start the program you will receive error messages.
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Is there a painless way to swap the 20GB for the 120GB?
Any suggestions I have for this are quite painless, just a bit time consuming. Involves partitioning, moving data, removing drives, installing drives then reinstalling the OS & programs. Of course, ghosting would be alot easier, but I figure if you're going to change drives install everything fresh, unless you're really pressed for time. Ghosting will probably take less than an hour. Installing everything fresh (OS, updates, drivers, applications, transferring data, etc.) will probably take 4-5 hours including planning & preparation.
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All I have is a backup or two on the new 120GB. So I could spare it as new.
Backups are important. I would not suggest you wipe out your backups, unless you have another drive to store them on.
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But then, is the 20GB going to be too small for backups?
Probably. All depends how much data you're backing up.
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What size should I shop for for the backup if my primary will be 120GB?
All depends on how much data you're backing up. Look on the 120 now and see how many gigabytes the current backups use and try to project how much data you think you will add in the future.