There's a strong possiblity that this problem may be no more than your tapes having been written using a different block length. You can force Arcserve to use different block length by making a registry change under the tape engine key. Refer to the CA support site for more info in the key.
Chris Horvath - True Data Storage Networks
Premier CA VAR, Arcserve Training Center, StorageTek Dealer
arcserve_pro@yahoo.com