Copy a tar file to tape?

I'm in the process of revamping my backup procedure.

Currently my backup procedure consists of using tar to compress and save
 a set of directories directly to tape.

I'd like to change that so that I create a compressed archive on a
removable usb hard drive, and then copy the compressed archive to tape.

The problem is, I can't figure out the best way to copy the archive to tape.

I've tried using 'dd' to do the copy ...

dd if=backup.tgz of=/dev/tape bs=512

... but I get a "dd: writing `/dev/tape': Invalid argument" error
message ... and "st0: Write not multiple of tape block size." shows up
on the console.

Any suggestions?

Thanks!

david