Which means that some of your 4GB is set aside for the iGPU, so Windows might have less than 3.75GB of RAM to run in. This is not ideal. As
@titan says, you need 8GB (or more). I try to fit 16GB in each of my older PCs, to give Windows + Programs room to "breathe".
If you check the Memory section in Resource Manager (accessed from Task Manager) it should give an indication of 'Hardware Reserved' memory (used by the iGPU).
On my i7-4770K rig using iGPU graphics, I lose 342MB system RAM to 'Hardware Reserved'. This is not a problem because I have 4 x 4GB (16GB) DDR3 RAM installed.
Whilst you've got the Memory tab open, check the amount of 'In Use' memory. If you have very little 'Standby' memory, you're running out of system RAM for Windows to work in. As
@titan says, this will lead to more page file use (on the boot drive?) which will slow things down.
When you download a torrent, the file is gradually built up from thousands of small pieces. Regardless of which hard disk you use for the initial torrent file, if the drive is not empty, the saved file will be heavily fragmented and scattered across the drive.
Reading back a heavily fragmented file from hard disk, takes longer than reading the same file stored in one big "lump" on contiguous disk sectors.
Similarly, writing a large file to hard disk takes longer, if there is no space large enough to save the data as one contiguous "lump". You have no control over where the disk controller saves files.
A brand new empty hard disk will allow faster reads and writes, because it is easier for the driver controller to write/read data to/from continuous sectors. In addition, outer tracks run at twice the transfer speed of innermost tracks, because there are double the number of sectors per track.
When the drive starts to fill up and you delete some files, it becomes fragmented. Read and write operations take longer, as the heads skip back and forth over the tracks/sectors, picking up hundreds/thousands of small (fragmented) data blocks to complete the file.
Add to this the problem of Shingled Magnetic Recording, where the drive is forced to read back multiple tracks and shuffle data around when saving new blocks, and you've added to the delays.
If you are still seeding a torrent after downloading a file, the disk drive will be busy serving other peers with dozens/hundreds of file fragments, slowing down your copy process when you transfer the file to another drive.
Given the limitation of your system (2-core CPU, low 3.1GHz clock speed, limited RAM, external SMR drives), I'd recommend closing all unecessary programs before transferring files and disconnect from the internet. Your torrent manager, web browser, etc., will all be consuming CPU clock cycles, system RAM and disk access time. You need to reduce all other activity to a minimum when testing disk to disk copy speed.
SMART doesn't always give all the information you'd expect or hope for, unless you're prepared to run a long SMART test on each drive. A long test can take hours and is best done over night when you're asleep. With any luck, the test will have finished when you wake up.
https://www.tenforums.com/tutorials/163843-how-check-drive-health-smart-status-windows-10-a.html
A more easily comprehended SMART test with GUI is available in Hard Disk Sentinel:
https://www.hdsentinel.com/
This is an example of a hard disk with serious problems.
I'd also suggest running CHKDSK /F /R on each drive to check for file errors and bad blocks.
https://www.howtogeek.com/1033/how-to-use-chkdsk-on-windows/
On your system drive, run SFC /SCANNOW.
https://www.howtogeek.com/222532/ho...-system-files-with-the-sfc-and-dism-commands/