It sounds like your violating the encription rules or not following the process to swap encripted cards. Also there is a limit on the size of a card the BB will support based on the BB type and OS level. This link may help with the issue
KB11667-Switch Media Card Wizard
BlackBerry Device Software version Media card size limit
BlackBerry Device Software version 4.2.0 Up to 2 GB
BlackBerry Device Software version 4.2.1 Up to 4 GB
BlackBerry Device Software version 4.2.2 Up to 4 GB
BlackBerry Device Software version 4.3.0 Up to 8 GB
BlackBerry Device Software version 4.5.0 Up to 8 GB
BlackBerry Device Software version 4.5.0.81 Up to 16 GB
BlackBerry Device Software version 4.6.0 and later Up to 32 GB