Extended SMART test will not complete in Synology NAS

I have Synology 12 bay NAS, DS3615xs.

Drives 1 to 5 are WD40EFRX-68WT0N0 firmware 80.00A80.

Drives 6 to 12 are WD40EFRX-68N32N0 firmware 82.00A82.

The issue I have is drives 1 to 5 will NOT complete an extended SMART smart test via the Synology NAS.

I can leave the test running for days and will not complete.

Extended smart test with drives 6 to 12 all complete fine in under 10 hours each.

Could this be a firmware issue with drives 1 to 5? Do they need a firmware update?