Ext USB backup hard drive disappears and then reappears

I have a customer with 4 500GB WD MyBook ext USB hard drives they use for backup.  It doesn't matter which drive is connected, they all do the same thing.  They will randomly disappear and then reappear.  The drive is assigned drive letter F.  It seems to happen when the drive is read.  If I open the backup program and initiate a backup the drive disappears.  If I wait for the drive to reappear and then kick off the backup manually it will work fine.  

Things I've tried:
Different USB port on the server
Different USB cable
Turning off AV
Disabling non-essential software
Script that "wakes up" the drive by adding a small txt file to the drive before the backup runs
Disabled power management from shutting off USB devices

Any help is appreciated, thanks guys.
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dbruntonQuid, Me Anxius Sum?  Illegitimi non carborundum.Commented:
Try a powered USB hub between server and hard drive instead of directly connecting between server and hard drive.  Could be that the drive is getting insufficient power at times from the server.

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If the disks use an external power brick, try another.
Carlos IjalbaIT Systems DirectorCommented:
Try it in another Server or PC, if it has the same behaviour then it's the USB device, if it works fine, then:

Uninstall the USB device using the freeware "USB Deview":


An then plug it again to reinstall the USB device on the server.
computerconceptsAuthor Commented:
Thanks for the suggestions guys!!  The problem appears to be with the AC adapter we were using.  The customer couldn't locate any of adapters from the other 3 identical hard drives so they are going with a new set of backup HDDs that aren't powered.
I'd suggest they rather get a USB dock, and then they can use any internal SATA disks in those docks for the backups. That is much cheaper and you are much more flexible.

USB disks powered just by the USB ports I've found to be unreliable too, as very often they don't get enough power from the PC. Those powered by external power bricks are more reliable, and if the Power brick fails it is easy to replace. You only need to get one with the same voltage and at least the same Wattage.
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