This thread was resolved by CommVault support. It turns out that a mix of 64 and 512KB block size LTO4 media was the culprit. When the SAN-attached NetApp filer tried to overwrite the OML of a 512KB LTO4 tape, it would generate an I/O error. The source data path for these NDMP SAN-attached NetApp filers is configured for 64KB block size for the LTO4 media. Apparently with this shared scratch media pool using a range of block sizes for LTO4 media, we would consistently run into this. All that had to be done was to check the "When content verification fails" checkbox of the "Overwrite Media" section of the "Library Properties. Since we did this, the auxiliary copy has been running fine.