Most can, but read the unit's manual carefully. Almost all offer to let you establish a RAID array in their case, but don't make it clear that you are not compelled to do so.
However, some DO require that you use some form of RAID. Their built-in "smarts" are set to make themselves appear to the host computer that they are one "drive", and don't ask for details. Such units may only offer you the choice of which type of RAID array you want - 0, 1, and 5 are common choices. Some will also offer JBOD, which is not really a RAID array but is often included in the systems to manage RAID. JBOD stands for "Just a Bunch of Drives", and it is a way to connect several different HDD units so they behave as one large "drive".
Even those that make RAID available but do not force it, may have a default configuration to create a RAID array from the HDD units you mount inside. So read the manual to be sure you know how to set up the configuration you want, and not just whatever default it comes with.