HDDs are cheaper alternatives to expand storage. They are compact, but not embedded in the device, hence making it slightly risky for the files to be lost or physically stolen. They are very affordable and are used by a lot of DJs for extending their storage without paying large amounts. SSD is faster, safer and shock resistant since it is placed in the motherboard. It can be accessed and edited faster than a standard HDD and is most preferred nowadays. The prices have been going down, but an upgrade is still not as cheap as an HDD. Now, to upgrade the storage, either of the two can be utilised, or a combination of the two.