thanks for your response adamsmith, I'm trying this now, just taking the time for the copy paste of the 600M.

//update 2: Two strange things happened and it seems the problem might be solved.

First: while I was waiting for the upload of the library folder on my USB key on the old computer, I used the spare time to download the Zotero plugin for firefox on the new computer, and I updated my login and password in Zotero standalone. And...suddenly it worked ! Zotero standalone filled with what seems to be most of my data. I filled the "relative path" box and it also seems to find painlessly the pdf files on the hard drive.

It comes as a bit of a puzzle and makes me wonder if I may have lost a lot of content. The simple explanation is that the data is actually synced in Zotero's cloud, but I wonder how that is possible as the file size of the library on the computer is 592 Mo (size 500Mo, size on disk 592Mo), whereas on Zotero's cloud it says 279.5 Mo. That makes me fear that I lost 300 Mo in the process. But a dozen random tests have not reavealed any missing item. Down to the onenote links that I had inserted in some of the notes, which is very impressive. If you are still reading (sorry that it's so long) : would you know how I could have a check on missing content ? some kind of "properties" report that would tell me the number of items ?

Second strange thing: on the old computer, it seemed to drag on forever to copy paste the onedrive zotero library file. it could not beat 6ko/s. So I turned off the internet and it went much faster and completed. But the real strange thing is that the disk space taken by the library on the USB key is now 1,6 Go ! (it says: size:500Mo, size on disk:1,57 Go) i.e. 3 times the size on disk that it was taking in the hard disk, in the onedrive folder. The onedrive folder was fully available offline so I don't think some info was missing there.

So that makes me wonder: how useful is it for me to keep this library folder from the old library ? I am not sure I want to risk undoing the current "miracle" of the synced data base to find out if I am able to use that old library folder. However I would like a sense of comfort that the current set up is both complete and reliable for the future. What do you reckon ?

many many thanks if you followed until here and for your initial advice

Ok, thanks Adamsmith for your feedback. That's what I intend to do. Would you by any chance know if I can have some data on the number of items in Zotero so as to compare between the old and the new computer in a big picture / quantified way ?

Great, thanks ! sorry I should have checked the FAQs myself. I haven't been able to compare with the old computer as I had to hand it in before I could perform the test, but I have a good feeling about the current situation. Thank you again Adamsmith for your good instinct and fast reponses.