Network problems when copying large files. Reason?

Hello. Network: 1000Mbit switch(A) connected to the gateway, file sharing, and another 100Mbps switch(B) connected to user machine. All unmanaged switches soho. Before that there were no problems until you need from sharing the user's machine to copy the image weighing in at 1.5 GB. As soon as it starts up, none of the users can not go online until the end of the operation, because all that is for the uplink of switch B can't ping, including the gateway. Changed the cable, put the other switch(same 100мбитные) instead of B - all also. Copying in the opposite direction is no problem.
It remains to check only the switch A. it May be that the Gigabit link from filehosting fills entire input buffer in 100мбитный downlink to switch B?
I hope more or less clearly described) Prompt please where to dig?
July 2nd 19 at 17:06
2 answers
July 2nd 19 at 17:08
Solution
Thank you all, suspicions confirmed. Replaced Gigabit "plow" on 100Мбитную) All was adjusted.
July 2nd 19 at 17:10
Prompt please where to dig?

1. Continue to localize the problem, removing one after another "suspects" in problems of components.
2. To find the cause.
3. To eliminate.
4. Profit!

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