Network problems when copying large files. Causes?

Question

Computer networks

Hello. There is a network: 1000Mbps switch (A), to which a gateway, a file hosting service and another 100Mbps switch (B) are connected, to which user machines are connected. All switches are unmanaged soho. Before that, there were no problems until it was necessary to copy an image weighing 1.5GB from the file hosting service to the user's machine. As soon as copying begins, none of the users can access the Internet until the operation is completed, because. everything that is behind the uplink of switch B is not pinged, incl. and gateway. Changed cables, put other switches (the same 100 Mbps) instead of B - all the same. Copying in the opposite direction passes without problems.
It remains to check only switch A. Could it be that the gigabit link from the file sharing service overflows the entire buffer at the input to the 100 Mbit downlink to switch B?
I hope I described it more or less clearly) Please tell me where to dig?

Answer 1

Thanks everyone, my suspicions have been confirmed. I replaced the gigabit "plow" with a 100Mbit one) Everything was fine.

Answer 2

Please tell me where to dig?

1. Continue to localize the problem, removing one by one the "suspected" components in the problems.
2. Find the cause.
3. Eliminate.
4. Profit!