Downloading large files and defragmenting

I have not used a disk defrag utility on a Mac since System 8.

As far as I know it is not needed.

In a word, no. Do note that if you use something like Carbon Copy Cloner to make a bootable backup of your boot volume to a bootable, external HD (preferably FireWire) and check it to ensure that it's viable, you can erase the original, reverse the process, and defrag in one operation.

Since you're a newcomer to the Mac, see these:

Switching from Windows to Mac OS X,
Basic Tutorials on using a Mac,
Mac 101: Mac Essentials,
Anatomy of a Mac,
MacTips, and
Switching to the Mac: The Missing Manual, Snow Leopard Edition.

Additionally, *Texas Mac Man* recommends:

Quick Assist.
Welcome to the Switch To A Mac Guides,
Take Control E-books, and
A guide for switching to a Mac.

As long as your internal disk isn't near full, fragmentation of big files shouldn't be a problem.

If you have at least 15% free at all times, OSX won't have to scatter the parts of a big file much.

But if the drive has been consistently below, say, 10% free, and you've downloaded or copied several big files, Baltwo's suggestion of cloning to an external HD and back should help.

Also note that when the drive gets near full, things will slow down noticeably, and if it continues to fill, files may get corrupted, apps may crash, and eventually your Mac may crash and not restart.

So the main thing is to always give OSX some "breathing room." 🙂

Thanks for the feedback. Good computing.