HDD is the secondary storage device which is used to store all the file systems. Qlik stores everything as a file system and uses its own file extensions to store data, so that it can be retrieved faster.

Being a Qlik consultant, you should know that the data is not only stored in QVD (qlikview datafile) but also in Qlik Sense application which has an extension of .qvf. So you need enough available space to create a dashboard. Qlik also generates extensive logs, and for that also you need space on hard drive.

Thus, estimating HDD is also important; you should have sufficient hard drive to store all the data.