if I were you, I would stop playing around with the Diagnostic test, although if you still have the ability to exchange it for another one, now might be the time to do it if you feel uncomfortable with the diagnostic results. I haven’t tried playing with the diagnostics as I’ve read another post that the user never recovers from his full diagnostic test.
My best guess is that after copying 600GB of data, your device starts to scan your data for media, thumbnailing your photos, creating index entries for DLNA, finding all your mp3’s for itunes etc. This process is very intensive and usually take a couple of days and sometimes weeks to complete. This is probably the worst time for you to play around with the Diagnostic testing as you are competing for CPU time on a device that runs on two underpower 600Mhz processors.
You have three choices after copying 600GB of data…
wait for the media scan to complete which will take a couple of days… although if you add more data or move the data around, the scan will continue…
turn off your media serving per shares, turn off twonky, turn off itunes server, turn off DLNA, until you have completed your data copy. Then turn on the media options and let your device index and scan all your media at once.
SSH into your device and issue the following two commands
This will stop the device from scanning your media, and wonky and itunes server should be turned off on your web/ui.
After copying all your data, reboot your device and your services will restart.