[SOLVED] Inconsistent write speed from HDD to HDD

[hotsaucebg3]

I am transferring large files(tv series) from one HDD to another over LAN(From Windows 10 to Ubuntu 20.04 system's samba share). At first 2-3 minutes the transfer speed is consistent at 105MB/s, but then something happens and it drops to random 20-40 MB/s(sometimes even 5-10MB/s).
What could be the issue with this?
Both hard drives are 4TB Toshiba P300 5600rpm 128mb cache (yes, the cheapest ones I know).
I have turned on the windows feature SMB 1.0/CIFS Client, but I got the same result.
I have tried with another Network adapter, same result.

 

[rgd1101]

issue is it a hdd.