External HDD unmounting unexpectedly An externally-powered WD My Book Essential, 3TB, which I use for Time Machine and other file storage, recently began "ejecting itself" from my iMac. In an attempt to repair the disk from Disk Utility, I came across these logs:

kernel[0]: jnl: disk1s2: replay_journal: from: 99262464 to: 99508224 (joffset 0x2bae000)

kernel[0]: jnl: disk1s2: journal replay done.

fseventsd[47]: Events arrived for /Volumes/My Book after an unmount request! Re-initializing.

fseventsd[47]: creating a dls for /Volumes/My Book but it already has one...

diskarbitrationd[15]: mds [38]:23343 not responding.

kernel[0]: USBF: 670537.390 AppleUSBEHCI[0xffffff802439f000]::Found a transaction which hasn't moved in 5 seconds on bus 0x26, timing out! (Addr: 3, EP: 0)

kernel[0]: USBF: 670543.393 AppleUSBEHCI[0xffffff802439f000]::Found a transaction which hasn't moved in 5 seconds on bus 0x26, timing out! (Addr: 3, EP: 0)

kernel[0]: USBF: 670543.798 The USB Family found a device at  port 2 of hub @ 0x26000000 with a bad USB device descriptor (0x0, 0x0 )

kernel[0]: USBF: 670549.396 AppleUSBEHCI[0xffffff802439f000]::Found a transaction which hasn't moved in 5 seconds on bus 0x26, timing out! (Addr: 0, EP: 0)


This began happening shortly after I've been accessing a movie file from the drive to encode and burn to a DVD from iDVD, but I wouldn't expect this to be the issue. Each time the drive unmounts itself, the error when not ejecting a disk properly appears. Note that in the log, the third entry from fseventd, I believe the "events arrived" was me attempting to repair the volume. 

What could be sending the unmount request? diskutil is. The problem, I believe, lies in the journaling. (see update, it's not this)

Update (for clarification also)

I know that repairing the volume requires the partitions to be unmounted, I meant it's ejecting by itself. Just repaired the volume from another Mac and it checked out OK...must be the Mac that uses it for Time Machine.

I'm convinced that it's this error message that is the issue, but I can't figure out what it means.

kernel[0]: USBF: 670537.390 AppleUSBEHCI[0xffffff802439f000]::Found a transaction which hasn't moved in 5 seconds on bus 0x26, timing out! (Addr: 3, EP: 0)


Update 2

Just finished an archive-and-install of the iMac in question, but I'm still getting the same logs and can't access the files. I can't blame it on a corrupt USB driver anymore, so I really am at a loss now.
This is symptomatic of a hardware failure in most cases. You'll want to isolate the various components:


disconnect all other USB devices
try a new cable
try a different port on the Mac
try that drive on another Mac (or another drive on your Mac)
try another power source for that drive


It's likely the voltage supply to the controller chip in the drive (or the chip itself is faulty or getting power spikes from the drive motor) so I'd consider backing up that data and contacting WD to see if they have newer firmware or if more power is needed to run that drive.