Tested on 3.5" hard drives with capacities of 2TB, 4TB, 6TB (WESTERN DIGITAL), 18TB, and 20TB (SEAGATE). For the first three, the case fits ideally and has a little margin to easily remove the hard drive; for the higher capacity drives, this ease is lost (obviously due to greater thickness - we are talking about millimeters). The case is for STORAGE and to prevent the hard drive from getting dusty, and DEFINITELY NOT for protection against drops, magnetic fields, etc.

It unbuttons a little difficult but maybe it's for the best (so the hard drive doesn't open and leave at any moment). Also, internally and only on the top part, it has a cushion to absorb vibrations. Good and useful product!

It's not bad but it's not perfect either. For its price and for small trips, it's okay.
An ideal case should have more pads, space for silica, and be airtight. This one doesn't have any of those, but I haven't found any case that meets my requirements, especially within this price range, so it's fine.
For frequent transportation of multiple hard drives, I would recommend:
https://www.skroutz.gr/s/9293102/Icy-Box-THiki-Prostasias-Aluminium-Protective-Case-For-10x-2-5-3-5-inch-Asimi-IB-AC628.html

I prefer some other cases that open like a "book" and have shock-absorbing pads inside (top/bottom/left/right), but for some reason, I can't find these cases anymore in the market. These are the best ones I found in this price range. It has a pad/"cushion" on the lid and I added another one on the bottom. It does the job, but it could be much better. I should mention that I frequently transport HDDs. For someone who just wants to keep them in a drawer, they do their job just fine.

Ears.