I finally received my USB stick (Verbatim, normally a good brand). Amazon has never been so slow!
Unfortunatly things are as with other sticks I already tried : LOADING appears on the display, instead of a countdown.
I have tried (and retried, and reretried...) every thing I could : 4 different sticks (all USB2), with sizes from 512MB to 32GB, FAT32 and FAT16, the 3 USB inputs, different procedures, different .tfd files, ... Nothing works.
I'm now pretty convinced that the problem is elsewhere. Perhaps USB drivers should be reinstalled or something like this, but I have no access to the operating system of the Toppy. The ethernet connexion doesn't work as the Toppy doesn't really start, and USB is ignored.
I also tried to connect the hard drive of the Toppy to my PC to explore its content (sorry for playing the doctor

I think I'll give up, and consider my SRP as finally dead. Thank to all those who responded. You did your best to help me.