I would recommend using a document management system. There are free open source ones.
https://www.google.com/search?q=open+source+dms seems a good start for you.
Of course, these systems reply on the OS for the storage and on you for the backup of all your files. I've never seen a backup system that couldn't make an image copy or even an incremental copy of a disk with too many directories or files. Since you're mentioning external drives, let me tell these are unreliable for storage and need extra care for backup. Why not use internal disks in your PC?
I have a collection of books, articles, & papers numbering into five digits XX000....that I have amassed over the last ten years. Till now I have used dedicated external drives using Windows filing systems. Iv got to the point where directories are too long and accurate backups are impossible
##-What I want in a database
1/ A simple interface with simple functions
2/ [Ideally] allowing me to plug and play the external drive on other computers and still have access to the database information/sorting system
3/ To have all the materials on one directory level so I don't have to worry about access or copy problems
4/ To be able to categorise into subject matters and assign individual files to multiple categories
5/ To be able to use simple entry tags, name, date, book/article etc
- Portability is important
- It's for private not business use
- It needs to be reasonably priced or free
Ps. Even if your responses do not cover ALL my needs I am still interested in answers relating to products