Usually, downloading goes on until it reaches 0 MB on Disk C. (Typically, there is about 700 MB free n my Disk C before the FS updates are downloaded; therefore, the updates files are larger than 700 MB).
Naturally, with 0 MB on Disk C, the system goes wrong (although, it will not always hand or crash).
In order to bring the system back to life, I simply delete the updates files. (And my recycle bin is located on Disc D, as well as all internet or other temp files, so that deleting itself does not cause a problem).
An FS expert has advised me that it is not possible to relocate the temp updates files to another disk.
Note, that there NO WARNING SIGN anywhere that in order to let FS work properly, a customer should reserve a couple of GB on their Disk C!
Anyway, it seems that there is a very simple solution how to avoid the Disk C overflow: while downloading updates, FS should check that the remaining space on Disk C is 50 or 100 or whatever MB, and if it is less, it should stop.