Why do SQL databases become unreadable after EFS decryption?

Question

SQL Server

Windows server 2008 R2 standard
MS SQL Server 2008 R2

database files (mdf and log) are encrypted using EFS.

The need to upgrade the server has come and therefore I started decrypting the databases:
but after decrypting EFS, the SQL databases become unreadable?
1. I detach the database
2. I decrypt the database
3. I do attach and I get an error SQL Server Error 5171: MDF Is Not A Primary Database File
i.e. base is corrupted

Answer 1

Make an unencrypted backup.

server upgrade

Deploy it either on the old server or on the new one.