You are all unintendedly treading on new ground. MacOS High Sierra introduces APFS (Apple File System) to Mac computers. APFS replaces Mac OS Extended (HFS+) as the default file system for solid-state drives (SSDs) and other all-flash storage devices. Kaboom! APFS is still having issues recognized flash memory units using other legacy formats. Don't get smug if you are using Windows or Linux, both are introducing similar and compatible flash formats this coming year, Apple was first to the punch. These are all part of new formatting techniques, long overdue, far more efficient and offering maximum usage of flash memory which does have a shorter life cycle than mechanical drives. Likely the owner of the Mac, when last performing a system update, didn't examine a choice of formats for his Mac to recognize. This cannot be corrected after the fact, yet.
Merely plugging a USB Flash drive into an APFS formatted system can screw around with the data contained on the drive, making files invisible to all users.
Adding to the problem, is the fact that MP3's placed on a Mac or opened from any auxiliary drive or source on a Mac, alters the MP3 adding an additional data fork, creating havoc, sometimes turning those files invisible.
That was the bad news. Now the good news, and life as we know it isn't over. There are solutions and work arounds that are simple, and more efficient than USB thumb drives.
There are many file storage sites (clouds so to speak) that offer free storage, sometimes of limited size, sometimes for limited periods of time, but all are basically letting users try out services they want to sell the consumer. Do a bit of searching, you can easily find hundreds of these file sharing sites, examine a few to see which would be best for you, upload your files that you want to share and pass the keys (authentication methods for access) to anyone you want to be able to download those files.
DropBox, which started as a file sharing site, is now a full blown collaboration site, with many users able to simultaneously work of the same file. Usage determined by paid subscription plan.
As a very musical family, we consistently pass files in progress between family members and friends, at time with more than a couple of hundred files being in use. Keeping track of which files are to be accessed by who, latest modifications can be a nightmare without an up to date database with everyone doing their best to for updated contributions. We often use Apple's iCloud accounts that come free with 5gb of free space with each device aquisitiaa.
