SSD and all USB drives LEDs constantly flashing.

This seems to be an old problem with no clear resolution. With my PC booted but not logged on my SSD (C: drive) and two USB connected drives have their activity LEDs constantly flashing - it appears, in time with the system timer ie a regular flash about every 1 second. The only drive that is not affected in this way is a USB drive connected to the USB port on my BT Home Hub. Trying to track it down with Process Explorer but so far no luck. Problem exists whether I'm logged on or not. Anyone got any thoughts?
TIA
PC is
MoBo:ASUSTeK COMPUTER INC. Z170-P Rev X.0x
UEFI: American Megatrends Inc. 0705 02/23/2016
CPU: Intel i3-6320 @3.9GHz
16 GB RAM
Windows 10 Pro 1803 (17134.285) (Genuine)
Belarc reports the following for the drives:

BIWIN SSD [Hard drive] (256.06 GB) -- drive 0, s/n 161607901029, rev O1026A, SMART Status: Healthy
Maxtor 3200 USB Device [Hard drive] (500.11 GB) -- drive 3, s/n 2CAHGJTJ
Seagate Expansion Desk SCSI Disk Device (2000.39 GB) -- drive 2, s/n NA4KRZ2X
TOSHIBA DT01ACA100 [Hard drive] (1000.20 GB) -- drive 1, s/n Z5G5D5VNS, rev MS2OA750, SMART Status: Healthy
and this excludes the USB drive connected to the HomeHub

Board: ASUSTeK COMPUTER INC. Z170-P Rev X.0x Serial Number: 160265951800267 Bus Clock: 100 megahertz UEFI: American Megatrends Inc. 0705 02/23/2016

There will be a sheduled task to defrag drives when idle could that be it?

Thanks Samuria. Task Manager shows nothing obvious; I've disabled auto defrag but the problem persists. The physical disk drives I could put up with but I don't want unnecessary read/writes to the SSD as it will shorten it's life. I'll keep searching.
Thanks

I was going to try booting into safe mode (from within windows) but Windows didn't like it. Still looking.

Task Manager > More Details > Performance > Open Resource Monitor ( bottom left next to Fewer Details) > Disk and see what is reading or writing to the drive.

That's what's so bizarre. The only drive showing as having any activity under "Disk Activity" in ResMon is my C:drive yet all 4 physical drive graphs on the RHS are showing activity in their Queue Lengths and, apart from the USB connected to the Home Hub, all drive LEDs are flashing in time.
BTW finally got into Safe Mode with Networking and the drives were quiet ie no activity.

The search continues.