thats a vender specific name. my samsung has that as 0. take as look at raw read error rate, hardware ecc recovered and seek error rate. usually drives that are about to go have those numbers shooting up. I read the attributes listed a dir and listed the attributes again. on a questionable drive it often goes up by the million's of errors in just a few min.
do a benchmark on the drive when booted from another drive see if the read test has any dips in specific spots while the drive is not mounted.
its not unusual for a drive to have a stair stepped look where it starts at 50mb and dropps to about 30mb or 80 to 40. It should never have severe drops like 50 to 20 or less then back up if its not in use. I got the 80-40 mb from my 3.3 year old (29020 powered hours) drive. I have also seen bad hdd controllers and system board controllers cause it to start @5mb and occasionally spike up to 20mb
you may want to consider a full drive read scan. If you question it read test it. I have seen many a dell pass the smart and confidence test yet fail the read test. we often use smart details and benchmark results for when we know drives are going to fail but havent gotten an error code yet to get them to rma drives. Some of it is just getting to know whats normal for your drives so you can do trending.