Unable to safe boot on iMac16,2

Lexiepex wrote:

Does you have the same issue when the Lacie is disconnected ?

With the LaCie disconnected, I cannot backup Time Machine, since that is what I use for that purpose. With it still disconnected, I can do a safe boot and can accomplish the disk utility first aid. By first opening the Time Machine preferences in system prefs, I can select that LaCie drive for backup and have it appear on the desktop, but not without doing that. Otherwise, it doesn't appear on the desktop by itself.

Lexiepex wrote:

Do you have Fusion Drive ?

Yes, I have a 1.02TB fusion drive.

Lexiepex wrote:

Can you login with a guest account ?

I disabled my guest account a while back, for fear it was causing some other problems. If I initiate one, what characteristics do you want it to have?

Lexiepex wrote:

-- I suspect the Lacie disk. Also I am in favour of a disk dedicated for TimeMachine, no other partitions on it.

What happens if you disconnect the Lacie, hang on a correctly formatted other external disk (1 partition), and use it for TimeMachine?

My LaCie disk is dedicated only to Time Machine backups, with no other partitions. I do not have another external drive to attempt your proposal.

Lexiepex wrote:

-- Guest account: nothing special for the guest account. do you have the same issues ?

I reinitiated the guest drive and conducted a safe boot successfully and used disk utility to accomplish a first aid too.

I assume the LaCie drive has a lot to do with these issues, and I have used the recommendations made several months ago when I first encountered some of those same kind of issues. I, in fact, purchased another replacement LaCie drive that until now worked fine without problems. Using those instructions, I switched Time Machine OFF, I DESELECTED/REMOVED the LaCie as the backup drive, then I DELETED the apple.com.timemachine.plist before restarting the iMac and reselecting the LaCie drive as the backup disk. When the TB Backup starts up it hangs up with the phrase PREPARING BACKUP. I have waited at least three times for over half an hour without its ever finishing the backup.

I just uploaded the update, macOS Sierra, to see if anything changes. Nothing did regarding Time Machine backup or the LaCie drive itself.

My Time Machine started backup after about six hours in PREPARING BACKUP status. It has now caught up and has backed up at least twice since then. For now, I believe my iMac is running properly, so the discussion posting is no longer relevant. Thanks for helpful hints.