So, because it’s frustrated me a bit. I ordered a 1 TB SDD to get a comparison and et voila it suits perfect.
It seems that the “measurement tool” I used was not very proper. I measured the 2TB HDD just with a calliper, it is the 9,5mm version. Even though the article infos in the shop where I bought it told me it’s 7mm (funny enough, on the package of it isn’t any info about the size).
Nicole, my sincere apologies. You were totally right!
The idea of a reasonable priced storage wasn’t that good, obviously! But 1TB should be enough for a while.