Unless the operative systems for computer,s the so called image, that is the O.S of a linux receiver, DOES NOT reside in the HDD but in the Flash Memory.
So, you won't loose anything from your HDD when you install an image, because you'll write only the eMMC, the flash memory.
Only, beware of the fact that past versions of some images, immediately after the first boot, asked you for eventually formatting (and so, ERASING) the HDD. But I haven't seen this message any longer in the last years and in the images I've tried. Surely, it is not prompted in our BH and OBH.