I have just installed Linux Mint 17.3 KDE and have been very pleased with the installation. However, dolphin will not write music files to USB sticks or my Digital Music Player (Fiio X 1). Other file managers write perfectly. Can anyone help?

Dolphin doesn't actually write anything and the particular content doesn't matter either - nor does the bus type (ie. the actual device type)
I assume it's less a USB stick but more likely some USB player which you cannot access because it's not a mountable block device but only accessible via mtp:// ?

=> http://community.linuxmint.com/software/view/kio-mtp

If that's not the case, you'll have to specify "how" dolphin fails/refuses to write into the mounted directory (error message?)
Also post the output of "mount" and annotate the line that refers to the device you'd like to write into.

Luebking

The kio-mtp solution worked a treat, thank you very much.