Controller circuitry for a serially-recording magnetic tape drive which is capable of correcting writing errors by rewriting the portions of the data which have been erroneously recorded. Write circuitry in the controller breaks a conventional data record up into one or more blockettes, each of which is assigned a unique sequential blockette number which is recorded on the tape along with the data. After each blockette has been serially recorded sequentially by blockette number on the tape, it is immediately read to check whether it has been properly recorded on the tape. If the blockette has been recorded improperly, the read process directs the write circuitry to rerecord the blockette information at the tape position then under the write head. The recorded blockettes may be out of sequential order since the rerecorded blockette may be located several blockettes after its initial erroneous writing. The data is placed back in sequential order by the read process. As blockettes are read from the tape, they are placed in a buffer memory in the controller. The read circuitry then transfers the data to the host computer system in the order specified by the blockette number and therefore reconstructs the data even if blockettes have been rewritten out of sequence.