Posted 08 January 2013 - 05:16 PM
USB2 drives are certainly slower to work with but I don't see them any different to internal drives. Speed is only part of the issue. Ease of maintenance, reducing wear and tear and file recovery are other factors.
First of all, partitioning or organizing to suit usage. If you have all your "library" files, eg home videos, in one dedicated partition or whole drive and they are badly defragmented, the HDD head is jumping all over the place to find the bits when you play them back. Since the videos are never changed, defrag once and you are done. Each video will be on the same track or adjoining tracks, reducing wear and tear on the HDD. You only need to defrag occasionally to accomodate new additions.
A drive or partition with frequently changed files can have various scenarios. If the files come and go quickly, there is no point in defragging. If the files hang around for a while and are being accessed, it's worth defragging. If these files are kept in a different partition to "library" files, you can quickly defrag (or malware scan) without processing a bunch of files that don't need it.
Smart defragging the OS drive keeps the startup files together and at the start of the disk. May or may not be noticeable in startup speed.
My observations (because I defrag and see what is going on) are that defragmentation rapidly escalates, particularly with mixtures of large and small files, and when you get low on free space. You find that there is not enough free space for the large files so they are fragmented from the start. Using a smart defrag maintains the largest possible free space which reduces future fragmentation.
If you are scanning for malware and files are in numerous parts, the HDD head is having to skip all over the platter to find the bits. Same deal with wear and tear.
In the rare event of a drive crash, the more advanced recovery algorithms look for recognizable file headers and footers. If the files are not contiguous, that is completely defeated.
A contiguous file is quicker to be read but given today's speeds, that is not a big deal. As pointed out by others, there may be speed limitations elsewhere.
Defragging puts some wear and tear on the HDD. You limit the amount of defragging to suit the content and what has recently been added.
Like running any utility, if you periodically observe the process, you get a feel for what is normal. That makes it easier to spot unusual activity.
The benefits of defragging are quite small but it's such a trivial task if you do it to suit the drive usage.