Full Scan Will not Scan Removable Drives

I have a Toshiba Satellite L755 running Windows 7 Home Premium (currently) Avast Free Antivirus 17.8.2318.
Yesterday I my laptop ran its scheduled full computer scan, which I ave set to scan “all removable media”. This normally takes between nine and ten and a half hours, most of which is spent scanning my 500GB Toshiba Canvio USB hard drive. I also connect my 16GB flash drive during the scan.
However, when I checked on its progress about four hours and twenty minutes in, my external drive showed no sign of activity, and Avast showed zero percent progress. As I have previously gotten inaccurate progress indications corrected by going to the Antivirus screen and clicking on “Full Virus Scan” again, I did so, and still saw “0%” displayed. I canceled the scan, and clicked"SCAN HISTORY", where the scan was listed as having been completed in “1:20:44”, testing 115.6GB,instead of the usual 1.1TB. Normally, my laptop would go to sleep when the scan is completed, as I leave my laptop alone aside from occasional progress checks.
Therefore, I checked my inbox, then restarted my laptop; after, when I was unable to open the Avast UI. As I have been unable to open it before, I restarted again, and was able to open the UI.
However, when I started a full computer scan, the problem recurred (supposed run time: 1:16:14, 116.0GB tested), so I tried using the “Repair” function from Programs and Features.
After restarting again, I ran the same scan, which supposedly lasted 1:25:43 and scanned 117.7GB. The scan still showed zero progress when it came time for Backup and Restore to make a backup, over fifteen minutes after the supposed completion. I tried to open Backup and Restore to stop the backup, which I expected to fail, but everything except Avast and the cursor froze, including the Start Menu. After waiting in hopes of anything changing, I forced a shutdown, then started my laptop again.
My final scan attempt supposedly lasted 1:13:02, scanning 116.0GB.
These results my be caused by scanning until getting to my external drives, hitting a problem, and getting stuck there, but I don’t know what that problem is.
Can anyone help me?

Avast, do you not care that your product is not working? Why won’t you help me?

Hi, could you please try to scan just the 16GB flash drive and 500GB USB drive separetely via Targeted Scan?

Yes. I removed “all removable media” this week, and scanned them that way. The separate scans took longer, though, and I would rather not have to scan them separately. Is there a way to fix the problem, not just work around it?

Now Windows Backup keeps giving me an error code (0x81000037), which, while a different, recurring issue, I can’t solve this time. Since that is Microsoft software, I doubt I will find much help here. While clean booting and turning Avast’s shields off sometimes solved that in the past, it hasn’t this time. I also have tried running CheckDisk in safe mode, and the two methods combined have been enough to fix the problem together in the past, but not this time.

See if this helps:
https://www.pagestart.com/win7br0x8100003701.html

I already scan before backing up; I wish that would prevent the problem.