For those running an external USB hard drive for imaging / backup purposes, there is every reason to suppose fragmentation builds up as fast as on an internal partition.
The obvious concern-- although we are already severely limited in I/O speed by USB2.0 limits (to a lesser extent by USB3.0) the major problem is defragmenting an external USB HD partition.
Under USB2.0, this becomes almost a disincentive to regular maintenance-- defragmentation must be done overnight and into the next morning, or not at all. More often than not, defragmentation is not done at all.
Has anyone an opinion whether the quickest way to resolve fragmentation is simply to reformat the external drive before a backup session, and be done with it in a few minutes?
I don't worry about my NAS backups - it (supposedly) doesn't need defragging - plus I don't care about it as it's only for backups.
But my backups to USB after about 6 months or so, that is what I do, format the drive and the next backup is a full one.
As you state, it is faster to format and do a fresh backup than to do a defrag.
Not that a frag'd backup device is that bad anyway - theoretically the USB backup device will never be used (famous last words) and if it is, it'll be a one-off read and you are so grateful the data is there you wouldn't care at what speed to get it back at.
Plus even if the USB device is getting frag'd, as a backup unit, it's only getting written to, so any head seek times to write data is so inconsequential compared to read seek times that it really shouldn't play too much into the overall unit performance.