Thanks for your reply.
Ever since I moved from XP to Win7 (a few years ago), I switched from Windows’ built-in copy-paste function to a small freeware called TeraCopy ( http://codesector.com/teracopy ). Although I am comfortable with Win7, I found the new search / file copy features in Win7 to be inferior to the XP way.
I can probably rule out AV checking. I use Microsoft Security Essentials, and my experience has been positive.
I keep my backups in multiple media / devices. So, before file copy into the USB Flash drive Cruzer, I did an identical copy-paste into my Western Digital external hard disk 1 TB, which took around two hours. That was one-third the time for the copy to the Cruzer.
Also, I have copied files into an earlier SanDisk Flash drive 64 GB. Naturally, the copied volume was about half of the copy into the new Cruzer 128 GB. But, the copy happened significantly faster.
So, wondering whether about 6 hours for a 100 GB copy operation is ok or not.