Inconsistent Volume Contents After Breaking Errored W7 "Dynamic" Mirror
August 11, 2018 12:39 PM   Subscribe

I had a pair of two 2TB 3.5" SATA drives mirrored through Windows 7's logical volume management (i.e. a "dynamic disk" mirror managed in diskmgmt.msc). After noticing errors on one of the physical disks, I remounted the disks as individual volumes, and the contents are not consistent. Only the errored disk is up-to-date. What gives?

The background: lately apps running from or using the mirror have been acting strangely, prompting me to check its status in Disk Management. It was flagged as "At risk" with one of the underlying volumes reporting errors. So, I "broke mirror" in the volume management options so as to be able to mount the underlying drives separately.

After doing so, I was unhappy to notice that the drive without errors did not seem to be up to date.

Only the errored drive seems to have data added or changed within the last few months. This is not what I would expect from a mirror, even a degraded mirror (and regardless of whether LVM, software RAID or hardware RAID).

I could see Windows deciding to stop synching the mirror to discontinue use of the errored drive, but instead it seems that it stopped synching and relied on the errored drive only, discontinuing use of the healthy drive.

I disabled caching on the errored drive, re-imported it and gave it its old drive letter back and things seem to be working well enough to hopefully get the up-to-date data off of it. My tentative plan is to use an external drive for that, and then swap out the errored drive with a new one, copy the data back and then resync to the surviving disk to have an up-to-date mirror again. My instinct is not to run CHKDSK on the errored drive to investigate further without getting the data off first, in case I'm on the threshold of a total read failure.

My question is mostly about what happened here: why isn't there current data on the unerrored disk? If syncing the volume across both disks was disabled due to errors on one disk, then why keep using the disk with errors instead of the healthy one? And why did this happen silently?
posted by snuffleupagus to Computers & Internet
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