The ones that you want to monitor especially are the Reallocated Sectors and Pending Sectors. reallocated Sectors are not necessarily a problem as long as the number stays small and stable, but a large number or a continually increasing number are a bad sign. Pending Sectors can be intermittent, but they are always working as they indicate there are sectors that may not be read correctly if another drive fails and a rebuild is necessary. However they not infrequently get cleared when the sector is next written so again not necessarily fatal if they only happen occasionally. The CRC count is one to monitor as it indicates a connection problem (typically cabling) rather than a disk problem, but they can still cause a read or write to fail causing Unraid to disable the drive. CRC count never gets reset - it only increases so keeping it stable is what you aim for.