Symantec Drive Encryption has the ability to encrypt hard drives, both Internal Fixed Disks, or External, or Removable Disks.
Decrypting Primary Fixed Disk before decrypting Secondary Fixed Disk can result in data not being accessible.
Fixed Disks are disks that are connected directly to the system itself, inside the computer case.
Decrypting Fixed Disks should be done with special consideration. Although these disks are separate fixed disks, they are grouped together with Symantec Drive Encryption for Disk management, such that if decrypting the Primary Fixed Disk is necessary, decrypting the Secondary Fixed Disk, first, is necessary.
If the Primary Fixed Disk has been decrypted and the data is critical and no backups are available, please contact Symantec Support to troubleshoot further.