This invention provides a means for protecting previously written data tracks on magnetic tape from excessive trimming due to tape expansion or contraction caused by changes in environmental conditions. The width of tape media varies based on environmental conditions. If data tracks are written on a magnetic tape at one environmental condition and then data is later written to the same tape at a significantly different environmental condition, the previously written tracks can be over written to the point that the data on the previously written tracks can no longer be read. This invention measures the width of the data band on the tape at the time the data is written and records that information for later reference. If data is to be written to the tape at a later time when the environmental conditions may have changed, the new width of the data band is measured and compared to the previously recorded width when the adjacent track was written. If the difference in widths is determined to be large enough to compromise the adjacent track, the write operation will be modified or disabled in order to protect the previously written data
