A storage apparatus includes an interface and storage circuitry. The interface is configured to communicate with a plurality of memory cells organized in multiple memory blocks. The storage circuitry is configured to produce a given readout by reading data from a group of the memory cells in a given memory block, using a given read voltage, to calculate a given zeros-ones imbalance level of the given readout, based on the given zeros-ones imbalance level, to check whether the given readout level is zeros-ones balanced or unbalanced in accordance with a balance criterion, and upon detecting that the given readout is zeros-ones unbalanced, mark the given memory block as suspected of being unusable.