We have all four drives chewing up tapes on  regular intervals. To make a
rough estimate I would say that this happens  once or sometimes twice  a
month and all drives are effected in the same way. I have no idea if this
is the normal wear  due to lots of mounts.
We always check out the tape which caused the problem and in most cases we
have a technician change parts of the drive or even the whole drive.
The age of our tapes is different. We exchanged a lot of them to new ones,
but also still  have  tapes which are 3, maybe 4 years old.