Old iMac Hard Drive not readable as external

Question

My friend's iMac would not boot. I ran TechTool Protogo using a thumb drive with TTP 9 to test the hard drive. It reported several hundred failed sectors on the drive, so I recommended we replace it.

Before replacing the hard drive in the iMac, I booted off an old hard drive from an old laptop using a USB to SATA adapter to make sure the iMac would boot off another drive, in other words, I wanted to make sure the problem was in fact the hard drive. It booted fine, although I could not access the damaged internal drive as it was encrypted with FileVault.

I went ahead and completed the hard drive replacement, putting in an SSD and connecting the damaged internal to my USB to SATA adapter. I expected I could get the password for the old hard drive later to copy any data I could save from it.

I used internet recovery to install the original OS that came on the iMac to the new SSD, and then upgraded to High Sierra, which is what my friend originally had on it. The iMac now boots fine.

The problem I have is the former internal drive will not mount using the USB to SATA adapter, rather the iMac reports it is unreadable. I also used the adapter to try and mount the hard drive on a MacBook Pro, but got the same warning,

Is there something related to FileVault that would prevent this drive from working outside the iMac? I had researched this earlier, and thought I was okay. Any advice or help would be greatly appreciated.

1. iMac Model: 21.5-inch, Late 2013
2. OS installed on failed disk... High Sierra
3. OS installed on new SSD ... High Sierra
4. OS installed on MacBook Pro ... Mojave

I did find a thread on here with a similar problem and tried fixing it with GPT commands in terminal. I wonder if I have made it worse...

Please help me. I really wanted to help my friend and though I got her iMac up and running, I really, really want to help her get files back.

------- Additional Information -------

The drive is a HGST SATA drive, model 5K1000-1000. The USB to SATA adapter is the CablesToGo #3050.

@benwiggy, When I booted from the laptop HDD, the iMac started up fine. A window appeared and I believe it said the internal drive was encrypted and asked for its password. I simply ignored it assuming I could access it later as an external drive and get the same prompt. When I tried to access it as an external drive, it said it was not readable and offered me the option to ignore, eject, or initialize. I chose ignore.

Terminal Commands and the results:

Bobs-MBP:~ bob$ sudo gpt -r show disk2
  start       size  index  contents
      0          1         PMBR
      1          1         Pri GPT header
      2          4         Pri GPT table
      6  244190635         
244190641          4         Sec GPT table
244190645          1         Sec GPT header


Bobs-MBP:~ bob$ diskutil list
/dev/disk2 (external, physical):
 #:                       TYPE NAME                    SIZE       IDENTIFIER
0:      GUID_partition_scheme                        *1.0 TB     disk2

Answer 1

Do a cold boot and enter the password for the encrypted drive at that time. FileVault 2 doesn't allow access to encrypted drives after boot if you don't provide the password then.

When FileVault setup is complete and you restart your Mac, you will use your account password to unlock your disk and allow your Mac to finish starting up. FileVault requires that you log in every time your Mac starts up, and no account is permitted to log in automatically.

See: https://support.apple.com/en-us/HT204837