Having trouble transferring files to my NAS

So I wanted a NAS. Thought building my own with a Raspberry Pi was a cool little project and learning experience. Had a few minor hiccups because I had little prior experience with Linux and no prior experience with the Pi. Got everything working eventually after searching this forum and Google. Thanks to everybody posting tutorials, guides and helping others.

Raspberry Pi 4b
OMV 5.2.2-1
Connected via Ethernet
External HDD (EXT4) connected to my Pi via USB3

MiniDLNA, SMB, FTP all work.
Streaming media files to my Android and Windows devices works just fine.

However I can't solve the problem of copying my media library to the NAS. The first couple of dozen files transferred fine from my laptop to the NAS, then the transfer suddenly stopped until Windows gave me an error message that the device was not responsive. The Pi couldn't be accessed through the OMV web interface or through Putty at this point, until I restarted it. I repeated this a few times and transferred more files over, but the transfer was aborted at some point every time.
Googled the problem and found threads that described similar problems, suggesting that this could be solved by using FTP instead.
So I switched to transferring the files from my desktop machine via FTP instead of SMB. At first I got constant speeds of 110MB/s, until the transfer was interrupted after a few dozen files too.
The error message my FTP client gave me was this:

Zitat

Copying files to remote side failed. Transfer aborted. Interrupted system call.

Which... doesn't really tell me anything to be honest.
I also found threads describing similar issues caused by the Pi's ethernet controller and I tested transferring files via Wifi, which didn't help either, I couldn't even transfer a single 2Gb file before the same error occurred.

Any suggestions on what else I can try to fix this?

Thanks for the suggestions. The HDD is a WD MyBook with its own power supply, not relying on the Pi to power it.
When I ordered the Pi, I ordered a heatsink along with it. It's a passive aluminum heatsink covering both sides of the board. The Pi does get warm to the touch though, which did concern me a bit. Will check the firmware.

/e
Turns out the firmware was already up to date.

Just reinstalled Raspbian and OMV on a different SD card. Transferred about 200Gb of files successfully and now I'm getting this error again.