We present a new data allocation method for probe-based storage devices that use multiple, simultaneously accessed parallel data fields. Our method uses blocks of data of unequal length for allocating a sector in the various storage fields. The amount of data stored in each field depends on the sector's offset from the beginning of the allocation round and on the storage field used. Numerical results demonstrate the storage efficiency improvement that is achieved by the proposed method. We show that this method can be applied to atomic force microscopy-based probe storage devices.