Looking at the
Seagate Adaptive Memory Technology
and scrolling down near the bottom is a diagram on the left showing at a high level the architecture.
The host is your PC. Since the encryption/decryption is performed in memory on the PC and all read/writes are encrypted, the NAND should have encrypted files which is controlled by Seagate's controller. The NAND probably uses
However, if you have accessed any sensitive data on an existing SSHD before encrypting the drive, any data will still remain unencrypted in the NAND even after you encrypt the SSHD until the Seagate controller overwrites that part of the memory with encrypted
data as part of its most frequently used data or programs.
The above is my analysis and should not be considered correct. You will need to contact Seagate to confirm my analysis and if encryption will negatively impact their Adaptive Memory Technology from caching frequently accessed blocks of encrypted data.