can anyone explain why when I copy some files to another hdd while downloading something on utorrent the speed suddenly drops and then goes back up once the file copy is complete? why would that even effect my download speed?
Each of the read/write heads on magnetic HDDs (i.e. non-SSD) can only perform one read or write operation at a time, so multiple parallel transfers are going to slow down the drive's transfer rate, especially if the files are fragmented. It's not the download speed that's getting slower, it's the speed at which the HDD is able to store the downloaded data.
also IRQ sharing can cause issues if ide/sata controler is on same irq.. but most times its heads on hdd going bad or the limitation of hdd functions at same time ( as posted by 2nd poster)