To make copying quicker from my hard drive to external hard drive and the other way around, I sometimes have several transfers windows together transferring multiple files/folders.
Does this method cause fragmented hard drive and it is better to have 1 transfer instead of multiple?
In practice, No. Theoretically, Yes.
No, because there are several factors that could cause a file to be fragmented, if you execute one or three copy dialogs at the same time shouldn’t matter.
Yes, because it could happen that one file from one dialog only write a file partially and the second dialog write the next file directly after it, causing the first file to be fragmented. However, given how many writes a typical computers executes every day, this could be ignored.
I guess the best solution (if you want to be 100% sure) is simply executing the copy as fast as possible to get the job done fast, and in the night run a defrag cycle.
In case you are running Windows 7 you don’t even need to this because it will run DEFRAG once a month anyway.
In general, copying multiples files won't make them copy quicker...
I imagine that all those files come from a single point (for example, one flash drive or one HDD) and go to a single point (one HDD).
In the HD-to-HD copy, if files are defragmented in the source, the head of the HDD will be positioned in one position and read through. If you choose to copy multiple files, the head will be positioned somewhere, read some data, them move to another place, read another piece, them go back to the first file, read some more, them move to another place, them....
Take a look at this question: Which is faster, copying everything at once or one thing at a time?
