Just to close the topic:
Official statement from Symantec, Engeneering is working to fix this but it isn't easy to do so don't expect it to be fixed until 7.1 or 7.2 at the earliest.
Problem comes from the fact that in 6.x, even when excluded, when the drives are enumerated, it sends some fake data to tape which it doesn't in 7.0 resulting in a code 90 as if the drive is empty.
Workaround: As Andy stated, instead of ALL_LOCAL_DRIVES directive, specify each drive individually (and its recommanded by Symantec and speed up backup) when possible.