I to have been having the same problems with both Ext Drives and a RIAD 6 Setup.
What really annoys me is this was deffenitly an Apple fault. As proven by the 10.8.5 Supplemental update.
In my case this has cursed me masive problems, and lost of working time.
This is because I had a SATA RAID 6 setup with 8 drives in 6 in the RAID plus two Hot Spares.
Before I new anything about this fault I can back to my Mac which i'd left on over lunch to find the RAID totally currupted. After may may rebuild attemps I thought the RAID was stable again. However to be sure I got two new 4TB Seagate Ext drives to copy to.
I copied across from the RAID to the first Ext Drive. When I got back and checked the copy the data amouts all added up. So I thought I had a safe spare copy, which was luckly because at that time the RAID crashed again.
I them decided to copy the copy to the other new Ext Drive to make sure I still had two safe copies of my data.
On doing this I was give some of Apples favourite error messages. Telling me that firstly the information on the first Ext Drive was currupted and second that the second drive had ejected during the copy and hence become unfixable by Disk utility.
So for me the outcome of Apples error has cursed me the lost of all the data on my RAID plus all the Coped data on the two new back ups which a brought just for this.
If this is not bad enough after wasting all day today doing the updates trying to rebuild the RAID and reformmating the two new ext drives. You would be mistaken as during this time three out of four of the internal drives in my 2009 Mac Pro unmouted and could not be seen by disk utiliy or even in the System Report.
After some time and a restart I had a massage come up saying that disk utilily was rebuilding two of these drives which I have RAID together. The other drive which was a time machine copy of my MAC HD was still no where. So I pulled that drive and put it into a drive reader though USB and it came up but again totaly currupted. So before I could put it back into the MAC I had to reformate it.
This means that this error has taken out eight out of nine drives. All of which I an still working on and do not now yet if the data is safe. Plus distroyed my RAID which again I do not now if I can recover the data from.
So well done MAC for yet another great week of working on repairs and not paid work.