Problem copying files with unusual characters in their names to the Galaxy S4 SD card but not to the phone I run Gentoo Linux on my laptop. MTP connection to my phone works well, and I can see both the phone memory and my SD card. I can copy files to and from both to my laptop with no difficulty. The laptop is exclusively UTF-8 filenames.

I was copying some music files from my laptop to the phone, when I noticed a series of errors with files whose names contained punctuation.

Specifically files called '08 - ¿Por Qué?.flac' and '01 - Visión: Bautismo En La Cruz.flac' would not copy over to any directory on the SD card.

They would copy with no problems to the Phone memory itself, and can be played from there. Files with names containing accented characters copied fine, for example '07 - Unción En Betania.flac'. Files named '0?8 - Por Qué.txt' and '?08 - Por Qué.txt' also caused errors, but a file called '08 - Por Qué.txt' copied normally

I assume this is problem with the SD card file system, and its support for Unicode. any suggestions for managing it. I can (and have) renamed the files, but that is only a partial solution. Anyone got any better ideas?
Yeah you´re right. The problem is caused by the file system of the SD card. Probably you use FAT32 as file system of your SD card which is using the OEM character set (Read more). And the OEM set seems to not contain some characters you need as you found out.

So you need a different file system on your SD card which is using UTF-8 character set (NTFS on your lapto does, and also the internal memory of your phone supports it (which could be an ext like ext4). 

But the best cross OS supported file system should be exFat. As you can see in the Wikipedia entry it supports UTF-16 character file names (just a variation of UTF-8). Also with this file system you have the ability of storing files bigger than 4GiB, which is not possible on FAT32.

So backup your files and format your SD card with the program of your choice (you can use even the basic windows integrated formatting tool). Then the problem should be gone.