Yes, that's the reason, but not the whole reason. Import duties are generally low and regulated by the WTO, and 4% is already a lot, unless the items are a weapon in a trade war. I don't know about SSDs but here's some info from the government of the Netherlands suggesting it's 0% for computers and parts. EU is a customs union so the 0% rate applies everywhere within it.
Customs NL - buying on the internet. Take into account additional costs such as VAT (btw), import duties (invoerrechten) and clearance fees (inklaringskosten).
www.belastingdienst.nl
However, there's an administrative procedure involved too, and it probably costs the same no matter if you pay a 0% or 50% duty. The cost varies between EU countries. Here in Slovenia, we pay around 3.25€ fixed fee for anything below 150€ that arrives from China (or the UK, for that matter). It doesn't matter that the customs office just needs to confirm that the Chinese seller has already charged, and paid, the VAT for us. And for stuff above 150€, it costs 28€. (Those costs are found in
this price list.) And that's probably why some less informed buyers order stuff, then refuse it when it arrives at the border.
Huh ... was that
after Brexit? I'd understand that if it happened before. But now, there are additional costs for crossing the Channel.