They may have been able to make this command they used:
cat /dev/md2 \
| pv -s 1888127576000 \
| grep -P --byte-offset --text 'FLV\x01\x05' \
| tee -a /mnt/storagebox/grep-log.txt
somewhat (or a lot) faster by adding a strings command (maybe with an appropriate length arg matching the grep pattern length) to the pipeline after the cat, and removing the pv call, be ause then only the printable ASCII characters woyls be passed to the grep, potentially heavily reducing the work it has to do, depending on the breakup between binary and text data (bytes) on disk.