FLAC On A Mac

I agree, the file should remain a FLAC file when you transfer it from your Mac to a thumbdrive. So I don't understand why the Marantz won't play it.
You know, this is likely unrelated, but I just copied my hi rez Rush and Milos Karagdelic FLAC files from my Mac to a flash drive. I also copied some Apple Lossless files to the flash drive. I wanted to see if my car could play the FLAC files, as it has a USB port. It turned out it could not. I wasn't really expecting it to. It occurred to me later though that the FLAC files may not be to blame, as I have literally thousands of Kindle book files on the same thumb drive, so maybe the car stereo got overwhelmed looking for audio files. SO, maybe we should both make sure there's not a lot of extraneous junk on our thumbdrives? I don't expect the Camaro to play the FLAC, esp hi-rez FLAC, but I do expect the Marantz to, of course.

I downloaded Decibel and it it was easy to use and now I can play the files, but not on the Marantz.
The display in the Marantz seems to be telling me that it will output only 44.1 khz. The Marantz is on loan to me from a dealer. I suspect that it may need a firmware upgrade( which is not free) to output 96 khz but when I called the dealer they were clueless and unwilling to let me make the upgrade (they had to borrow the piece themselves from another store). The Marantz also isn't capable of Air Play without the upgrade, so I can't test that feature. This is a real problem, as I otherwise am not very impressed by this piece. It sounds very laid back and uninsightful, no match for the firewire DAC that I use with my Mac Book in my home theatre system or for my PS Audio Digital Link 3 that I use in my two channel system.
Re: your car: I suspect that the USB will only play mp3 files or the equivalent

I deleted all the eBooks and the car stereo still crashed when it atempted to read the thumb drive. Oh, well.
I can't think of anything else to get the Marantz to play the files, so I guess you should wash your hands of it and try another device.

Does the Marantz read any other audio file types, such as aiff or wav, from the flash drive? If not, maybe the flash drive is formatted in a Macintosh format. Most of the audio devices I have tried to use with a flash drive require it to be formatted in FAT32.

I've taken my flash drive from my wife's PC to my Mac and there weren't any compatibility problems. I don't know how it was formatted though. It came formatted from Lexar.