External USB Drive Lost it's Format, and Performance data now shows it at near 100% activity

Question

From the Volume properties window within the "Disk Management" tool under Windows 10, I unchecked the "Allow files on this drive to have contents indexed in addition to file properties" checkbox. However since then I have been unable to access the drive, Windows Explorer tells me that the drive is not formatted!!

I downloaded the CGSecurity "TestDisk" fantastic tool, and it detects the MS Partition, and can view all the files within it. However, writing the partition data back to disk, does not resolve the issue of being unable to access the seemingly "Unformatted" drive... even though, according to "testdisk" everything seems fine... :

Disk \\.\PhysicalDrive3 - 4000 GB / 3726 GiB - CHS 486402 255 63                                                               
Partition                  Start        End    Size in sectors  
1 P MS Data                2048  7814057983 7814055936 [Archive]  
                                                                                                                                                                         
Boot sector                                                                                                               
Status: OK                                                                                                                                                                                                                                        
  
Backup boot sector                                                                                                        
Status: OK  
                                                                                                                                                                                                                                        
Sectors are identical.                                                                                                                                                                                                                            
  
A valid NTFS Boot sector must be present in order to access                                                               
any data; even if the partition is not bootable.                                                                                                                                          
(In hardware terms,  I have an old external USB 2.0 dual drive bay enclosure, set to span two drives as one, hence the reported 4TB size).  

Windows Task Manager shows the USB drive continually operating at nearly 100% , with Mostly Read speeds being reported

I attempted to delving into what was going on, using the Task Managers "Resource Monitor", and I do indeeed see the "System" process having the most Read's, but none are being directed at the USB drive. The Storage section simply shows Active time of 91.47%

I have tried disabling most of the Windows Services, but nothing seems to help with this constant access issue.

Any idea's / suggestions for investigating further ?

In the meantime, I'm going to try and copy (using "testdisk") the accessible files over to another USB drive. Worst case, I suspect I will have to reformat the drive and start from scratch.

Thankfully, it's my "Backup" drive - though I wonder what caused this mishap in the first place, was it simply that unchecking of the checkbox mentioned above ?

Has anyone else experienced such an issue, having done so ?

The reason why, I unchecked that checkbox, was down to a single file that refused to be copied, unless I compressed it as a 7-zip file, and as such, was holding up the remainder of my ad-hoc full backup process - I suspected that this Indexing functionality may have somehow gotten in the way, and for a backup, seems hardly necessary - hence why I disabled it... or tried to.

Also, is there a better way to find out what is really, causing the drive to operate at 100% usage; since the Standard Resource Monitor didn't identify any application / service accessing the remote drive, and neither did the SysInternals tools (procmon/procexp).

So how did the OS get information to lead it to report a utilization figure that it can't explain ??

I have Avast Anti-virus running on my PC, with the most thorough of settings enabled, and even ran the Microsoft Defender to double-check, as well as both "Avast's" and "KasperSky's" boot disk virus scan repair disks - neither picked up anything. Turning off Avast also didn't fix the original situation either....

But why, would the USB drive be reported at 100% activity ?