I suspect the archive is not complete.
Your command should have included the -M (multi-volume) command (to spread the backup over more than one tape). Just ejecting the tape may leave data in the buffer and not written to tape. You should first suspend tar with a <ctrl>Z, then unmount the tape device to flush the contents of the buffer to tape, then eject and insert a new tape.
Along with the -M option, you should also include the --new-volume-script=<script name> option, where <script name> is the name of a script to run at the end of a tape.
An executable script something like:
#!/bin/bash
<ctrl>Z # to suspend the tar operation.
umount /dev/st0 # to unmount the device and flush the buffer to tape.
eject /dev/st0 # to eject the tape.
Note: if these commands execute too quickly for the system to respond, insert a wait 5 command between the commands in the script. That will cause the script to wait 5 seconds before executing the next command.
#!/bin/bash
<ctrl>Z
wait 5
umount /dev/st0
wait 5
eject /dev/st0
That might work to unmount the device and eject the tape. After inserting the new tape, mount /dev/st0 to remount the device, then fg to resume tar operation.
To test the archive to see if you missed anything, tar --compare <archive name> to find differences between the archive and the file system.
