Hello JustTryinToGetItToWork, Extended Diagnostics is referring to running all the test on the system to see if they pass or fail.  Now for a TA flag 39 error it is best to remove all but 3 test media from the PV 124T and then run the Random moves test about 30- 50 cycles and see if the error happens again.  If the error happens again then in most cases there is an issue with the drive and the drive will most likely need to be replaced as it has a mechanical issue. Please let us know if you have any other questions.
I have a PowerVault 124T (LTO-4-120-1) LCD panel is throwing error TAFlag 39. Dell document says to run "Extended Diagnostics" What is that? How do I do that? (I don't see anything on LCD panel of device nor RMU that mentions anything of the sort)                                                                  
How do I run extended diagnostics?