Generally the format of a tape written in Retrospect consists of a header chunks of data and end of data markers (EOD). These EOD markers are crucial and must be written before a tape drive closes and finishes with a tape in the drive. During hangs or unexpected stoppages the drive usually doesn’t have a chance to write an EOD marker so the end of where the tape was left off is often left in this precarious state of missing an EOD marker.
Subsequent access of the tape, as in an attempt to update the catalog, could result in a hang or "resynchronizing (slow)" message. A hang during backup will result in a 'Catalog out of sync' message in a subsequent append to the same tape. This is because the catalog is written to in periodic increments in which case a hang or freeze would create a discrepancy between what’s on the tape and what’s recorded on the catalog. This is why you need to update the catalog after such events.
Due to the realities of EOD markers on tapes a crash or hang on a tape backup is pretty severe and usually hard to recover from. You often cannot continue that Backup Set. The ideal thing would be to start a new Backup Set altogether.
If the original hang and 'Catalog out of sync' message occurs on a third tape of a three tape series all data can be retrieved from tapes 1-2 and all data on tape 3 up to the point of the missing or damaged EOD marker. This means that if tape 3’s entire length of tape can be represented as points 'A' to 'Z' with the hang occurring at point 'L' that all data from 'A' to 'J' or 'K' roughly can be restored but none of the data at point 'L' or beyond will typically be accessible. If you want to retrieve data on such tapes in a consistent manner so that your restore does not always run up against the point of the tape where there is a missing EOD marker as in this example you can recatalog tapes 1 to 3 and stop the recatalogging process right around point 'K' on tape 3. This would avoid ever having Retrospect search for files beyond point 'K'.
Last Update: 14 Feb 2012