Erratic slow behaviour after changing HD

[readie]

About a week ago I changed my secondary HD from a 500GB to 2TB, copied and repartitioned etc.
I expected a slow backup perhaps on the next backup, but it seemed normal.
However since then on two days (I do a daily backup) the backup has taken 8 or 9 hours. Normally it is well under an hour.
They are incremental, not full, and no more data than other days, usually around 10GB or less.
I have pinned it down to the info "29/05/2018 00:55:47 :: Data (D:) (1.8 TB) 506.5 GB read at 17 MB/s [CBT] . . . 8:29:27" ONLY 17MB/s, whereas other normal days it is reading at 145MB/s on average.
Why should this happen on random days? Is there some strange fault with my new HD?
Any ideas where I might look for the underlying cause - and a solution?
Thanks, Bob

[Dima P.]

Hello Bob,

Can you defrag volumes on your new hard drive and then check the incremental backup performance? Also please clarify what backup target you are using. Thank you.

[readie]

(Sorry for delay - for some reason I didn't get an email informing me of your reply. I'll check my setting')
Thanks Dima. By doing some further diagnosis, I think I have pinned it down to Microsoft PHOTOS running in the background. I am also beginning to suspect that this was making some very minor changes to masses of photos/pictures, as the GB read by Veeam has now dropped from (e.g.) 506 GB read at ...(CBT) to more like a few GB read.
Certainly since switching off PHOTOS ability to run as a background task, backup times have dropped from hours to a few minutes, which is what they used to be. There are quite a few threads on MS discussions about PHOTOS hammering CPU and/or HD!
I might try a defrag anyway, as it is some time since I have done that.