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
 /etc/init.d/wdmcserverd stop
 /etc/init.d/wdphotodbmergerd stop
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.
Every time, i stop using my cloud during copying data for around 1-2days (600gb).
After i had transferred file to my cloud  my cloud will fail in Diagnostic Test. and transfer rate will decrease from 500Mbps to (50 to 250 Mbps)
after transfered all files (600GB), If i keep using it.
It will not allow me to download file from my cloud successfully (will stop during download / upload)…
i wonder if any problem with my folder structure or files ? thanks
By the way,  how do it know “my cloud” is working as below ?
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.
As I’m only another user on this forum and I’m only guessing at your problems.
You can hear the clicking and clattering of your drive while it is indexing, scanning, thumbnailing. Your drive will be very slow to respond and sometimes it just doesn’t respond at all.Â
With the new firmware, media scan is stopped if it detects a certain condition that none of us users knows, but after four times rebooting my device, the error message that I get is media scan stopped.
If you cannot immediately read a file back from the device, it could be the drive scanning your media. However I would phone WD tech support to get a better analysis.
