Antimalware Service Executable uses 100% Drive Access

Version 2004 (OS build 19041.804) For long periods of time this function of Windows 10 (a part of Microsoft Defender) takes up the whole of the spare traffic to and from the hard drive(s). To make that plain, in Task Manager/processes I see 100% at the top of the column and the % for ASE goes up and down to allow other processes to run but takes up all the spare - if others use only 0.8%, then ASE shows 99.2%.
It also uses up a lot of CPU, but not a lot of memory.
Obviously this makes the whole PC run extremely slowly, even stationary at times. It particularly affects Chrome but I don't follow why.
I have a single hard drive but it is partitioned to a C: and G:. Memory is 12GB;
Can anyone tell me how to prevent this happening? The process cannot be turned off, of course.
There don't appear to be any background scans running.

Check to see if you more than one AES listed in Task Manager. If you find another that is not doing anything per say then you likely have some malware masquerading as AES and is the cause of your issue.

If you find the above not to be the case then it is likely that real-time protection of Windows Defender is at fault. You can temporarily disable real-time protection to prove if this is the case. If this proves out then you'll need to discover what is causing real-time protection to remain active. Most likely it will be an application that is running some process that triggers real-time protection scanning. Adding that process to the Defender exclusion list will fix the issue.

Thanks for your reply Railtech; apologies for slow response. There is no 2nd ASE and switching off Defender made no difference. I read an earlier post on the same subject - shown below on my screen - and that led me to Task Scheduler (which I had forgotten). There were a lot of updates etc. scheduled to run in the daytime which I changed to run in the early hours and guess what? - it still made no difference.
At this moment on Task Manager I'm still seeing 100% Disk of which 92% is ASE.
I have found in Details the MsMpEng.exe and looked into its Properties but I've no idea what changes might improve matters.

Can it be that the drive is being de fragmented by a scheduled task?

A good thought, Megahertz, and it could have been a cause early this afternoon because I set up a defrag schedule only last week and it ran today. BUT it cannot be the explanation for all the previous times that I have experienced this problem.
However, earlier on I switched off the Real Time Protection in Windows Defender and that made an immediate difference. It has switched itself back on now but Disk Time has remained much lower. CPU use hasn't really changed but that was not a problem. This is strange because yesterday I switched Defender off altogether for a while and that made no difference.

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I'll wait 24 hours before marking this as solved, just to see if remains well-behaved.

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The problem is obviously NOT solved because the 100% Disk time was back again this morning. This time turning off the Real-Time Protection made very little difference. When I woke the PC from sleep this morning it did some very weird things which included changing the Window10 theme. I've not seen that before but I assume it has no relationship to the ASE problem.

Are you confident you have not been infected by malware? What you write sounds suspicious.