WD PR4100 repeatedly dropping attached USB drives

This started getting really bad a few weeks ago. I’d look at my desktop files on my MAC and notice that one or more of my drives attached to my PR4100’s of which I have 2 are no longer showing up. If I reboot they come back. I’ve switched usb ports, made sure power and data cables aren’t crossed or tangled, reset my modem, rebooted by entire system, changed cables and nothing seems to stop it. When it happens the PR4100 shows an unsafe device removal error for the attached usb drive. I’ve even reformatted the external drive. Both of my PR4100’s and their connected usb drives are mounted all the time as I have a virus scan which occurs in the middle of the night to check all of them. But this problem occurs during the daytime or just randomly. I’m at a complete loss. I should add that the drives that are attached are WD drives, MyBook Duo, Easy Store and MyBook drives. Anyone have any ideas?

I would check the power source to the USB drives - plug -power strip ?

I have many4 TB WD My books on my NAS systems and I need 2 drives to back up using 2 of the 3 USB ports as had a lot of 4TB My books and a 8TB NAS storage.

you also might want to test using a UPS power system so a power glitch is not the problem

as to USB drive type:
I am not sure but seems to me some MYbook firmware settings shutdown the drive down for no activity (easystore ?)

what format type are you using

It’s possible the power supply may be going. The drives have some age on them. I’ve tried both Mac journaled and ex-fat formats. I did make some changes to my Intego virus scan yesterday by turning off real time scanning and this morning all drives are still connected but I’m not sure how it might possibly be affected the drives losing connectivity. All drives are on a battery backup bank. Thanks for your response.

Today I changed all the power supply transformers and even used ones that will supply more watts and amps than the drives are expected to use at 12V. We’ll see as prior to doing this I was still getting the occasional drop.

After changing out the power supplies all seems to be doing well. I think that may have been the problem. Thanks for your recommendation!!!