I use several NTFS external disks with my Catalina Mac (for PC file exchanges). None of them are Spotlight-indexed, and that's fine as it is not necessary: macOS has no trouble seaching the disks without Spotlight.
All of a sudden one of the external disks stopped being searchable. macOS is unable to find any files. On my other external NTFS disks there is no problem with the macOS search function. (and Spotlight is not enabled there either)
(Rebuilding the Spotlight index is N/A as Spotlight is not used.)
I have the feeling something in my macOS is messed up regarding that particular external drive. Pehaps a setting that got corrupted? Is there such a thing as a .plist or so for a drive that can be deleted and recreated?
What could be the cause of the basic search function not working on an external USB drive?
Disk Utility, select the NTFS disk with issue, right-click it and click on Run First Aid... ... does it give any error or is the issue fixed?
chkdskon Windows platfrom, all report OK