Yes, to a degree, barbturtlo. what helped for me was to rebuilt the Spotlight index. as for granting it full disk access permission, that's not a problem. Apparently the "new" default is read only for everything, unless you have said otherwise. so, click on/highlight the external drive you want to change, and do a Get Info on it (command/Apple+I). you should get a long window. at the bottom of that window you will see sharing and permissions. to change that, click on the padlock on the bottom to unlock it. you will need to type in your main/admin user name and password, the one you used to login to your imac when you first started it up. you should see a list of users on the left, and the permissions on the right. change it as you like. then when you are done, click the padlock to lock it again and save your changes. it will ask you, again, for your user name and password, type it in and say "OK" and you are done.
Do the same thing for any other external drives you might be having trouble with. As for the Spotlight index rebuilt. that isn't too hard either. I don't know why Catalina has trouble finding stuff , but sometimes it does. The file/application might be there, but I can't see it or access it. Go to system preferences, then go to Spotlight, then click on the Privacy button. add the hard drive that is giving you trouble, then remove it by clicking on the (-) button at the bottom. try and find a file on that hard drive and you will see the cog spinning around showing you that Spotlight is rebuilting its index. when it is done, and the cog stops spinning, then the file will show up......
like I said, I don't know why it has trouble finding stuff, but sometimes it does, and this fixes that....
hope this helps you a bit
john b