This invention relates to a tape drive with an electronic buffer which temporarily stores data transferred between the host computer and the tape drive's magnetic tape. More specifically, during a write transaction, in which data is transferred by the host computer to the tape drive for storage, the present invention involves the buffer having an adjustable threshold, or "watermark", which must be reached by the data stored in the buffer before the tape drive begins the operation of ramping the tape up to its write velocity so that it can record the data stored in the buffer. To the extent that the rate at which data is sent to the tape drive from the host computer may vary, the adjustment of the watermark by the tape drive is for the purpose of locating the watermark that is optimal for the data input rate at any given point in time. It does so with its chief objective being the maximization of the tape drives availability to accept data from the host computer whenever the host is ready to send data. A secondary objective, however, is to minimize the number of mechanical operations of the tape drive in the interest of decreasing, to the extent possible, the amount of physical wear on the tape and the tape heads. The invention is also implemented in read operations and works in the same basic manner and has the same objectives as discussed in connection with write operations.