I've set up my raspberry pi (3B+) with a 2.5A power supply and an externally powered 2Tb HDD connected via USB.
Everything seems to run fine except for the sftp file transfers in which the device goes nuts.
I use filezilla to retrieve some torrented files from the raspberry and the transfer just plain stops for some seconds, then filezilla reconnects and continues the download.
At first I plugged a 2.5'' HDD non-externally powered disk and though that was an issue with power consumption, having the disk drain most of it and making the network unstable, but now, with an externally powered HDD, I have no clue of what could be happening...