Let’s say you have four computers you want to install Vocaloids on: you’d only be able to install and activate Vocaloid on 3 of those because Vocaloids can only be activated up to 3 times at one time. If you wanted to use your Vocaloids on another computer, you’d have to deactivate them on one computer to use them on a different one.
I will add this is why its incredibly important to deactivate your vocaloids if you arent using them or plan to move to another PC but of course, this doesnt mean you wont have those moments where you PC just dies and you cant do anything about. Do NOT format your HDD if you have activated your vocaloids on your System hard drive (the default highlighted option when installing). if you do not deactivate them you lose that activation for good unless you go through the effort of contacting yamaha or other companies, but even then its not promised.
HOWEVER.... there IS a way to potentially save your activations if your PC shits the bed. it is literally described on yamaha's website but not many people speak of it!
When installing your loids or editor you will have this pop up when activating.

we all know it, and you like dont click anything else, right? well, you will see one connection that is likely your WIFI/Internet/LAN card.
Authenticating your loid through that card will mean that if your HDD dies on you, you won't lose those activations, because that card hasnt died. of course, this requires some PC knowledge if more goes wrong. if your entire PC dies completely and that card if still good, you HAVE to install the same card on your new PC. at the very least, you then deactivate them then. but this is the BEST way to save your activation if you have issues with HDD, at least them its not tied to your system directly.
its important to remember how this works- the activator is using something to identify your PC.... so the HDD with the system on it? itll see that as the identity of the PC only. same with the internet card, that is its identity for the PC.
The reason why making your activation use the System HDD is dangerous is because you CANNOT (for the most part) move a system HDD to a entirely different new PC. unless the PC is identical in most hardware, you cant just slide that same HDD in. but an internet card, you are more likely to have compatibility.
This method however, assumes you use the internet/LAN card. in any case, if that card breaks or dies, or you disconnect it, you will have the same problem.. but it is less of a drastic thing to lose and less likely to fail. i suggest it as a solution because i know many who just pick the HDD. and this method will at least keep them detached from your HDD if something goes wrong, so theres a better chance of recovery. There is also the problem that i dont think all versions of vocaloid will show these options :/ but its definitely there for 2, 3 and 4 (?)
i will add that i have done this myself on this round w my new PC. there is no issue whatsoever using vocaloid right now, though i cant speak for the potential recovery if my PC does die lol, but i had it described to me by yamaha back when my other PC died and they seemed to speak as if that was the better way to activate if failure does happen.