Here are some things to check in case ThumbPrints Wiper™ can't remove all the thumbnail files on your device…
Windows evolves rapidly and the way it makes and protects thumbnail files changes. To be sure of wiping as much as possible, it's essential to use the most recent version of ThumbPrints Wiper™.
Click here, scroll to the bottom of the page and compare the current version number shown there to the number shown in the "About" dialog of ThumbPrints Wiper™. If the current version number as shown here on the website is higher, click the download link to get it.
Windows Explorer is the part of Windows that makes and maintains thumbnail files, therefore to destroy thumbnail files it is essential that all Windows Explorer windows be closed. Otherwise ThumbPrints Wiper™ and Windows Explorer wind up in a wrestling match over who has the rights to change a file. If that happens, ThumbPrints Wiper™ backs down because if it forced the file to be wiped, it could destabilize Windows.
Applications in addition to Windows itself can sometimes use thumbnail files, in which case they can place "locks" on them. ThumbPrints Wiper™ does its best to release such locks and wipe the files anyway, but under some conditions that’s not advisable. So if ThumbPrints Wiper™ reports that it was unable to wipe certain files, reboot your device and make sure nothing else is running, including any startup programs.
It's especially important to dismount any encrypted drives you may have mounted using programs such as TruCrypt or VeraCrypt. Failing to do so may result in your comouter crashing once Wiper runs.
Windows manages thumbnail files dynamically, opening and closing them on-the-fly. Sometimes all you have to do is scan and wipe again, sometimes more than once, waiting a few minutes between each. And please be really sure you have the most recent version of ThumbPrints Wiper™ and that you have closed all other programs.
Sometimes ThumbPrints Wiper™ will not be able to wipe some of your Thumbcache_xxx.db files even though all Windows Explorer windows are closed, and even after you reboot your device. This can happen when background processes running in Windows lock those files.
There are two solutions to this. The first is to restart your computer in "Safe Mode" and run ThumbPrints Wiper™ again. Starting in safe mode is easy in Windows 7, a bit more complicated in Windows 8, and a lot harder in Windows 10 (Instructions here or here).
The second solution is simply to wait a little while after a normal reboot (say, 10 minutes) and run ThumbPrints Wiper™ again. By waiting you give the background processes a chance to finish their work and unlock the files, thus allowing ThumbPrints Wiper™ access to destroy them.
After you do an initial wipe, run a new scan and look closely at the scan results. Not all thumbnail files will contain snapshots of sensitive data, and in the case of very small thumbnail images (say, 16 and 32 pixel sizes), it may simply not matter.
Also look at where the thumbnail file is located. Sometimes it will be part of "Program Files", inside the folder of an installed program from companies such as Adobe or Intuit. These are commercially distributed files and will not contain meaningful personal data. So if you get an "Unauthorized Access" error there, you can safely ignore it.
This is nothing to worry about (in fact, it's good). If you do a wipe, and then rescan, in the interim sometimes Windows will create new thumbcache files, which of course are empty because you have not yet browsed to any folders. You can safely ignore them, or wipe again.
Why bother with all this? Just get Merlin and make everything secure in the first place.