Library on two external hard drives (no iTunes)

can I alternate between two external hard drives without problems? When I connect one instead of the other, do I have to set something on Audirvana?

Might work okay-ish, if the content is kept identical, but Audirvana will notice the change even if the volume name stays same.
In short? Just don’t. The internal workings aren’t currently up to it.

Yes you can, one will be offline when unplugged and cannot be sync, but the plugged one will work as usual and songs wont play for sure on the unplugged one, but you knew that already I’m sure

Well, exactly, I’d call that a “no” more than a “yes”… You’ll see all the albums, some just won’t play, etc.

therefore, the best thing to do when using a new hard disk, or a second hard disk containing music other than the one present in the first one, is to delete the library and load a new one? Should I also remove the library database?

Depends on what you’re after, really? Perhaps easiest, get big enough disks and just store everything on one (or both). If you really want to “simulate” multiple libraries with different disks you could rename the database back and forth when switching or even use another user account (maybe - might hit license issues).
But, just hacks, no real solution.

It seems strange to me that nobody has two or more external hard drives with different contents to which the library connects and it seems strange to me that the developers of the player have not bothered to foresee the solution to this problem

Most of the users store the content on NASs or internal storage. Storing content on removable storage that is indexed and the index is residing elsewhere is always looking for trouble.

How so? If the mount point or - taking it a bit further - access path (think symbolic links) doesn’t change, there’s no reason for an external drive to be treated any different.

I certainly don’t have an extra terabyte of SSD storage on my 2016 MacBook Pro for my library, but a SanDisk 1TB SSD travels very conveniently and provides a very fast “cache” to which I sync the FLAC storage from my NAS.

Depends how the paths are stored in the DB.

I guess he won’t know until he tries if it works with multiple removable external drives.