I'd say if support says it's defective then send it back - but - if you feel there's more that can be done to troubleshoot then it's your choice.
Let's take a look at the symptoms
Doesn't stay active on your network for more than an hour ....
That could come from the device failing OR the network failing - try leaving the device idle and unused for an hour and see if it dies - or - open a command prompt and ping the unit's ip address continuously, if it fails under those circumstances I would say a defective unit is the problem.
On the other hand - if it doesn't fail or it only fails when you're trying to transfer data - you may need to look at the network.
I would assume that with a 600GB library you have at some point transferred significant quantities of data across your network and should have some idea of what to expect - so what I'll say is this - if you can transfer small files, but not large files, that would suggest network related issues.
Now I'll readily admit that I don't have 600GB music libraries - but - I've had my unit handling single >60GB files with no failures, and also transferring folders with huge numbers of files (171,000files, 11,000 sub folders, 14GB total) a process that can take upwards of twelve hours - now - if my files were all 3MB mp3 or similar, that would be close to 500GB, so I don't see why you should have problems based on the size of your library.
So - first you determine under what circumstances your unit fails, and it will in fact freeze if left to sit idle, then I would agree with tech support - oh - I've had my unit going on 20~21 months, just in case you're interested