TIME MACHINE NOW DODGY?

I used TM with OS 10.8.5 WITH EASE FROM 2015--01-01 with automatic reminder every 10 days.
MBP linked by USB to 3TB LA CIE drive.

Then updated OS to OS 10.12 in 2017-01 and since then TM seems erratic.
Seems to take many hours so I have had to change Energy saver to "NEVER" SHUT DOWN.

The latest fiasco 2017-04-17 was when TM decided to create about 7 backups one directly after another.

I had assumed that changing OS would not require me to do anything with TM.
But is there something I should have done to TM Settings?
Is there a way back to getting TM to its old habit of backup once every 10 days?
And not taking hours for each backup.

There is a weird issue that can happen with an "in progress" and if you google around there is a way to fix that (try "stuck on in progress")

But I also believe that how it is scheduled has changed (but rather unhelpfully, not the explanatory text in the dialog box). It now only backs up if user stuff has changed - can't recall the exact details but check Appleinsider or somewhere like that).

Couple of obvious things you've probably checked,
Disk isn't full, I know it deletes the old stuff off but in case that part's messed up and run disk utility over the disks, verify disk. If there's a problem with the disk it could cause problems.

Thanks for the various suggestions.
It now looks like Time Machine is almost back to normal.
That is to say, I now get a "time to back up" message roughly every 10 days.

But it still takes about 2 hours to complete the back up.
And it needs a click on "back up now" to kick start the process.

I guess I just have to live with these "improvements".

One thing I was not sure of was the supposed need to keep the LaCie back up drive more or less permanently connected to the iMac. Clearly not a practical approach with a movable lap top.
I only connect the LaCie when I have received the BACK UP NOW message, which may explain the slow process.

I assume that once the LaCie disk has been connected and the Time Machine process started, that we cannot use the MBP for normal other work. But I could be wrong.

Why not leave automatic backups on?

Time Machine is very well behaved and will back up as soon as you plug the drive in.

Seriously, whenever you are working at your desk/workstation whatever, leave the drive plugged in. Waiting 10 days between backups isn't really good practice and TM is very well thought out.

You can carry on working while it is doing its thing. Zero issues with that.

"Why not leave automatic backups on? "

Many thanks for that informative note.
I will try to implement that.

Thanks for all the contributions.
I have now stumbled on another "solution" to supplement the helpful ideas which have been shown.
When the MacBookPro was bought in MARCH 2013 it included in the box a small booklet
"Hello.QUICK START GUIDE Z034-6585-B" which on the first page showed a picture of the MBP connections.
These were Power, Ethernet, FW800, Thunderbolt, USB 3, SDXC slot, Audio in/out.

When I connected the LaCie 3TB back up drive, the connections available were [ fastest first ]
"USB 3.0, eSATA, FW 800, USB 2.0 ".

So obviously I connected the LaCie socket marked "USB 3.0 (USB 2.0 compatible) to the supposed "USB 3" socket on the MBP, expecting the claimed 5 Gb/s transfer speed.
It seemed to work OK, though not mind blowingly fast.

A day or so ago I looked in "About This Mac" and noticed that the hardware listed under USB was merely "USB 2.0 Bus" with max speed as 480 Mb/s.
This conflicts with the Apple "Hello." booklet.

It seems that the "About This Mac" data is more likely to be valid than the booklet.

I have now connected the FW 800 sockets on both LaCie Drive and the MBP laptop, and it seems faster [ claimed 800 Mb/s ].
It would have been faster to use the LaCie sockets for USB 3 or eSATA, but there do not seem to be equivalent sockets on the MBP.

Perfection is not of this world.

I am fairly sure none of the FW equipped MBPs has USB 3 (USB 3 and Lightning go together as in my mid-2013 MBP) . I think USB3/lightning superseded USB2/Firewire.

Where did you buy the machine? Sound like it has the wrong booklet in it.