External HDD rack unmounts when being written to

I have a small HDD rack (4 bay, 2TB max per bay, 3.5" Sata) connected to my iMac via USB 2.0.
I've had it nearly a year, and it worked fine always. I store my media on a 2TB WD (no partitions) drive that's in it, and documents on a 750GB WD (3 partitions) drive that's also in there.
However, in the last couple of days, it keeps randomly unmounting itself when it's being written to, and almost instantly remounts. The thing is though, it unmounts all partitions on all disks, not just one random disk having trouble with being written to. I can't understand how the whole rack dies when being written to.
Also, currently the only way I can replicate it is by downloading a file using something like Vuze directly onto the drive, and this makes it unmount itself within seconds of starting the download (have also tried several different files in case it was a corrupt file causing it). Copying normal files across doesn't seem to cause an issue. Finally, when it's not being written to/downloaded to or whatever, it can be on for days and never have a problem or unmount itself, whereas if I try and download something to it, it unmounts constantly.
Please help it's getting really irritating not to mention I'm worried about the risk of corrupted data.
Forgot to mention, disk utility says everything is in order.
Message was edited by: markytizz (re: disk utility)

Try doing a SMC reset, it might take care of it.
SMC RESET
Shut down the computer.
Unplug the computer's power cord and all peripherals.
Press and hold the power button for 5 seconds.
Release the power button.
Attach the computers power cable.
Press the power button to turn on the computer.
PRAM RESET
Shut down the computer.
Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
Turn on the computer.
Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
Hold the keys down until the computer restarts and you hear the startup sound for the second time.
Release the keys.

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External HDD rack unmounts when being written to

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