It’s possible, but my suspicion is that it didn’t really “fix” anything. I think it just moved things around, which ultimately is what defragging does after all. You might still have an issue that’s waiting to bite you later in a different way.
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Shuffling the problem around
Now, I’ll admit this scenario is a little out there, since this happened on both of your machines, but I’m still going to run with it because it is something that at least makes me uncomfortable.
I believe that you may have a bad sector on your hard drive.
It’s not so bad as to fail every time, but either it’s marginally bad, or it’s going bad so that right now, at least, it fails sometimes. I also believe that some files required during boot time were actually using that sector, until those files were defragged. Defragging simply moves files, or rather, portions of files around on your hard drive so that they are physically close to one another. It’s just a speed optimization; it doesn’t change the files’ contents in any way.
So it’s quite possible that some file was in fact using that marginal sector, and after being defragged, it no longer does.
Here’s why I think you still have a problem: that marginally bad sector is still out there. Maybe it’s now occupied by a file that’s never used. Maybe it’s free and it’s available to be used, in which case, it could bite you later when perhaps an important file tries to use that sector and your marginal sector turns out to be a sector that was going bad, and then it finally does go bad completely at a really inopportune time.
What I would do in your shoes is start with a CHKDSK /R on that drive; that will perform a surface test. Hopefully, it will find that sector and either fix it or remove it from any possibility of future use.
And of course I’m making the assumption that you’re backing up regularly. Right? :-)