On Sat, Mar 20, 2021 at 11:22:18AM -0400, Forrest Aldrich wrote:

I have a really large (3+ TB) volume I am copying to a BTRFS volume 
(both over USB) which is painfully slow.  In fact, it will probably take 
days to complete.   My goal is to use BTRFS on that larger USB volume 
(it's an 18TB drive)....

I don't suppose there is a way to convert APFS to BTRFS :)   I know, but 
I thought I would ask as it seems like others may have a similar query.

Convert relies on libraries providing the API to access the source
filesystems, I don't see anything like that for APFS so it would have to
be written from scratch. The linux-apfs project does not seem to be
active, last update 2 years ago. If it's just days of copying, I think
it's the better and safer option.