It isn't normal behavior of the software to not accept a valid passphrase when you are attaching the drive as a secondary drive on another machine. But what are the circumstances surrounding this problem? Is there a reason that you are attaching the drive as secondary to another machine, such as operating system failure or anything? That would give us some insight as to what could be going on.
Otherwise, you culd also have a lockout feature enabled on the passphrase if this is a client that is managed by the PGP Universal Server. You can set how many bad password attempts will lock out the user, and from then on, no further attempts to login as that user will work without manual intervention.
You could also try finding a recovery CD that works with the version of PGP Desktop that you have installed and try the passphrase on that to see if you can access the drive.