If you're trying to sort out a problem with a PC, and especially if it involves trying to diagnose a problem with disk or internet speed, then there's one thing you're going to need pretty quickly: a large file or two, with which to test the system. You can then try copying that file from one part of the suspect drive to the other, perhaps, or sending it by email to yourself, or whatever other action might help to diagnose the problem.
Being able to create large, random files on demand also has other uses too. For example, if you want to sell or give away a hard disk, you need to wipe it first. And the easiest way to wipe it is to overwrite the data on the disk with something that's not so important. By creating one or more large files of random, harmless data until the disk is pretty much full, you ensure that any important data which was deleted but was still on the disk (such as in the recycle bin) has been overwritten.
I've recently been investigating various programs that claim to be able to create a dummy data file on demand, and the best I found is called Dummy File Creator. You can download it from http://www.mynikko.com/dummy and it's free of charge. It's tiny, at just a 0.02 MB download for the portable version, and it's malware-free according to Virus Total and Web of Trust.
As you can see from the screen shot below, you simply fire up the program, choose the size of the file you want to create (in my tests it created a 20 GB file in less than a minute) and the job is done. If you're using the dummy files to test disk copying speeds and you want to ensure that there are no easily recognisable patterns in the file that could be cached in order to speed up the results, remember to choose the non-compressible option.
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