after changing external hard drives for back up, i can't store anything on that hard drive

If you are using it for Time Machine (recommended) then you shouldn't use it for anything else.

I have a separate USB Time Machine and a different USB HDD for storing miscellaneous files.

I do use iCloud for storing Documents, Desktop and Photos.app library etc. I can then access the files from my other Apple devices.

+1 Use one drive for Time Machine and the second for saving other stuff on.

If you have the new larger drive setup for Time Machine, remove the older smaller drive from Time Machine, then erase it and use it for saving other stuff on.