Dave,  thank you for your very helpful answer.  Here are the messages
from around that period (preceded by a timestamp added by a programmed
operator):

205859 ANR5230I 439: Starting Exit Machine DSMTAP0 to mount CARTRIDGE A12770.
205859 ANR5204I AUTO LOGON  ***       DSMTAP0  USERS = 163
205959 ANR5217I Dismounting volume A12769 - 1 minute mount retention expired.
205959 TAPE 0600 DETACHED
210055 TAPE 05A4 ATTACHED TO DSMSERV 0400
210059 ANR5301I 439: Verifying volume label A12770 on device 05A4.
210100 USER DSC   LOGOFF AS  DSMTAP0  USERS = 160
210100 ANR5313I 439: Mount for A12770 complete on device 05A4.
210421 ANR5364E Record sequence error on device 05A4.
210440 ANR5230I 440: Starting Exit Machine DSMTAP0 to mount CARTRIDGE A12771.
210440 ANR5204I AUTO LOGON  ***       DSMTAP0  USERS = 163
210540 ANR5217I Dismounting volume A12770 - 1 minute mount retention expired.
210540 TAPE 0400 DETACHED

The EXPORT continued for another 6.5 hours and reported no errors.  Tape
A12770 is the one I audited; the audit went fine.  QUERY VOLUME A12770
shows 0 read errors.  (And the volume was not set to unavailable.)

The server is at level 0.8.  (I am still eagerly awaiting an opportunity
to test a version of EXPORT with improved performance.)

Thanks again for your help,
Melinda