Can't finishing copying on Mac -- "Calculating Size" forever

I have a SanDisk Extreme 500 - 240 GB.  I’m attempting to backup files on my 2017 iMac.  The copy seems to have ground to a halt, and when I look at the folder currently being copied, I see “Calculating Size” – for about an hour now.

Is there a compatibility issue between the file system in the SanDisk and the one used by the Mac (MacOS Mojave)?  If so, should I reformat the SanDisk Extreme for my Mac?  If I do that, can I first make a copy of the files that came with it, and then transfer them back?  

I thought there was enough space.  Now I wonder about the number of files or inodes.  The copying was extremely slow through my stored mail items, which have lots of little files.  Now it seems to have ground to a halt, and it’s at a folder with an expanded CD database – probably 10 million discs, with a tiny file for each one (some kind of flat file database).  The total package isn’t huge, but it’s a lot of inodes.  I’m wondering if it may take days to finish “Calculating Size”, at this rate.  I don’t see an option for cancelling the file copy operation.  I’m not using the supplied software – just copying files from folders to the SanDisk.

Help?  This is unusable.  I think this is what happened 5 years ago when I bought the disk, and the reason I haven’t used it.  I would like to be able to use it, rather than have a wasted purchase.


Funny how the act of asking a question can lead to answers!  I just noticed a dialog that had been covered by other windows.  It showed the current copy operation – with 3.4 million files left to copy, and counting down.  Estimated time: 3 hours.  Apparently the "“Calculating Size” message was no longer correct, but just not updated by the OS.  Oh!  And there was a button to press to cancel the copy operation.  Back in business.  Okay.  I think avoiding stupid huge numbers of files may be the ticket.

But… any advice on file formats for use with a Mac?