Dean I haven't checked yet if multiman has this yet. Mind adding music playback for mp3 automatically start when you boot up multiman? Like as an opinion in configuration unless it already has it. Disregard if it doesn't, I'm not home at the moment.
Make sure the Playstation3 identifies your drive as "External" either in Video or Music/Photo tabs of XMB.
Your external USB should have at least one FAT32 formatted partition (max 512GB) - the FS should be FAT32 and NOT NTFS or fat32ext.
I have a 2 TB WD external formatted as FAT32 and it is recognized fine by the PS3. Why do you indicate a 512GB limit?
Also, using 1.10.13 or 1.10.15 the copy function freezes at 99% when copying most directories. I have only successfully copied one or two small directories in the 2.5-4.5 gig range. Source and target do not seem to matter. It will freeze on copy from disc to internal or from internal to usb. I think the copy completes OK as it always shows 0 time left and 4-8 MB left to copy. I have to power down the PS3 and restart when this occurs.
Also, being new to the scene I am looking into what SDK's are needed/available. Are you using one of the leaked SDK's or PSL1ght to do this work? Also I see source for 1.07 on the front page, but not anything newer. Do you have a publicly accessible repository somewhere?