HP Surestore DLT1 drives only read and write to new DLT IV tapes. DLT1 drives cannot overwrite tapes written by other DLT drives (i.e. DLT40, 80), even when the tapes are erased, the format will remain on the tape. In order to reuse media written in other formats, degauss the media properly, as DLT4000 format written by DLT40/70/80 drives saturate the media magnetically more than the Thin Film/MR head used in DLT1 can. Although media that is inadequately degaussed may still be accepted by the DLT1, residual data will likely cause poor performance and high error rates.

Why will DLT1 not overwrite DLT 4000 format media unless I degauss the media?

DLT 4000 format written by DLT 4000/7000/8000 drives "saturates" the media magnetically more than the thin film/MR head used in DLT1. It is possible that in the course of overwriting, some of the residual DLT 4000 format will interfere with the new DLT1 format, causing higher error rates or inability to read the new DLT1 format. Degaussing is a process of "randomizing" the magnetic particles (or dipoles) that are embedded in the media formulation -- effectively restoring the media to a "virgin" state. Degaussing is also sometimes called bulk erasing. Erasing a tape is not the same as degaussing as the format itself will still be resident on the tape.