OK. Scan should be over within an hour. I will try what you adviced me to.
If I consider scan will be over in one hour, it means that it took MB nearly 3 hours to scan 40.000 files on the NAS, amoung which 15.000 were new and, so, 25.000 had already been scanned. Does it look incredible or rather normal to you (knowing that my TS-231 NAS is brand new, connected by cable to my box and that my laptop is very close to the box) ?
Once the scan is over, I will try to scan with only one album more to see how long it take to add it the the collection.
Thanks again for helping me !