Samsung has a bad bug with all 840 EVO SSDs that causes a severe read performance drop to data that has existed on the SSD for an extended period of time. Yesterday they released a tool that will both apply a new firmware (prevent it from returning), and "refresh" the existing data, (restoring performance to existing data)
I applied it to two of my 1TB EVO disks, so far so good. non destructive if its a windows disk. didn't work at all with a ext4 partition on it till I cleaned that off and created an NTFS partition.
see techreport for details
Samsung's 840 EVO update fixes slow reads with old data - The Tech Report - Page 1
don't miss this if you own EVO's
I applied it to two of my 1TB EVO disks, so far so good. non destructive if its a windows disk. didn't work at all with a ext4 partition on it till I cleaned that off and created an NTFS partition.
see techreport for details
Samsung's 840 EVO update fixes slow reads with old data - The Tech Report - Page 1
don't miss this if you own EVO's