Backup disk does not auto-mount following logout-login sequence

The more I observe the behaviour of my system, the more convinced I am this problem involves both Keychain and system process response times during login AND startup.

A few times now I have noticed an odd thing that involves the Time Machine drive during startup. This external drive is encrypted. The initial password required to set up Time Machine was added to Keychain by ticking the "Remember me" box on the password GUI. The same password was then required to set up encryption. I also added it to Keychain.

The peculiar thing that happens is this : After turning on the Mac and boot up is nearly finished a window SOMETIMES pops up asking for the Time Machine password. If I enter the password I get a message something like : "Time Machine drive is already unlocked". The last time this happened I simply closed the password GUI without entering the password and I could see that Time Machine became available anyway ... as you would expect considering that the password had been saved to Keychain.

Although the same password is used for both access and encryption, I found two Keychain entries when I had a look using the Keychain Access App. Back when Time Machine stopped functioning I deleted one of these Keychain entries thinking that the duplication was a mistake. This did not fix Time Machine so I reformatted the drive and made a full backup from scratch. I re-used the old password. Now when I use the Keychain Access App I cannot find any password for Time Machine. Something is going on here that I don't understand, but it certainly sounds like it is a clue to my initial problem. HELP, I'M DROWNING.

This sounds very much like a wild guess and is lacking detail as well as no evidence. If I didn't know better I would think it was just a ploy to get me using a piece of software I know nothing about.

PS : I didn't know "party" went past 3rd grade !!

Yes please. From what I have found so far online is a general view that High Sierra has a few bugs, particularly in connection with mounting external drives. At logoff all drives are unmounted. At logon they not always re-mounted, even if they can be seen in Disk Utility greyed out. Response time of start up processes has been claimed as a factor. However, there are so many opinions and so called "free advice" linked to 3rd party software "solutions" it is impossible to tell what to trust. I need to hear from someone who actually knows, even if it means finding out there is a bug that will take time to fix

I have solved my problem.  It was a Keychain / Saved Password issue.  When some time ago I re-installed the Backup Disk and then Encrypted it, I decided to clean out the Keychain entry for that drive so as to take back MANUAL control of the password entry.  I now realise there must have been two "Keychain entries, one for the Disk itself and ANOTHER for Encryption, even though I used the same password for both.  However, I have concluded that I must have not removed all parts of the old Keychain entries.  As a result, it looks like this caused a conflict at logon.  

If the computer was STARTED or RESTARTED, the Backup Disk would mount and work properly, but at the same time a window would pop up asking for the Time Machine password.  If I entered the password, a message would then pop up saying the Disk was already mounted.  If I closed the password window without entering the password, Time Machine kept working, indicating that the passwords for mounting and encryption were being automatically retrieved from "somewhere".  It's as if the passwords were requested BOTH manually AND from the system, with the manual request being redundant.  By the way, I tested that the encrypted backups worked by deleting some unimportant files and then restoring them from the backup.

Now to the LOGOFF / LOGON scenario : in this case the backup disk would not automatically mount.  It had to be done manually Disk Utility.  Manual entry of passwords was, however, not required.

The solution has been to delete ALL Keychain entries for Time Machine, erase the Backup Disk and do a fresh backup using a new password.  I have elected not to use encryption as this just slows everything down and I don't have anything that important.  

A new search using Keychain Access does not throw up any entry for Time Machine.  I have concluded from this that Keychain must only save the encryption password.  With no encryption, there is no Keychain entry for Time Machine.  The disk mounting password itself must be saved somewhere else, or it is disguised under another name.  I am not certain about all my foregoing conclusions, but one thing is for sure : Time Machine works correctly, its password is retrieved automatically and the backup disk mounts automatically in all activation modes : START, RESTART, LOGOFF/LOGON and LOCK WINDOW/LOGON.

Thanks. At least my system is working as it should. I have done a bit of reading and thinking about encryption as an aside to this problem and decided it is not a good idea. The biggest problem I am likely to encounter is equipment failure. In this scenario, encryption could make data recovery impossible. I think a better approach is to duplicate the backup in another location using a newer drive and leave encryption to the CIA. Cheers.