It seems that the intended person is not available to answer your question.  I will try to assist you instead. What the person said was, you could right click on Windows Start button and select Disk Management.  From there, you check the drive C partition to see if it says encrypted on it.  If it does not, then it's safe for you to change, re-organize your storages.  If it does say encrypted drive, you will need to turn off secure boot in BIOS settings. From what I understand, the Inspiron 5400 AIO will support one M.2 solid state drive and one 2.5" SATA drive.  If your system has an M.2 already, your second drive will be a 2.5" solid state drive or vice versa.  I would not worry about disable secure boot.  Your system will recognize the 2nd drive without issue.  Just go to Disk Management and follow the wizard to partition and format the new drive. 
[NAME], Hello! Could you help me with that? I've read a few things talking about this but I fear I don't know enough about computers so I don't really understand. I would like to add a SSD to my Inspiron 5400 AIO as well, but what its this procedure you mention? Thank you kindly! 
Inspiron 5400 AIO, SSD as second drive? #2