Q: Problem syncing songs to iPhone when converting to 128AAC

I’ve been experiencing an extremely annoying issue with managing the iTunes library on my iPhone and thought I would ask if anyone here has a solution to suggest. In summary, iTunes is not reliably copying music from my iTunes library onto my iPhone when I select convert higher bit rate songs to 128AAC.

I'm using a HP laptop, with Windows 10. I'm running the latest version of iTunes and my iPhone 6 is running iOS 10.

My iTunes library is on an external HD, plugged into powered USB hub. The only option I have selected is to convert higher bit rate songs to 128AAC. I'm looking to sync 28.26 GBs of music and I have around 35 GBS of free space on my iPhone.

When I use iTunes on my computer – all of my tracks are found and play without a problem. The issue arises when I try to sync tracks from my Windows iTunes library with my iPhone – sometimes tracks copy and sometimes they don’t. Sometimes they look like they are coping but remain greyed out and unplayable. Sometimes they look like they copied but when I try to play them from the iPhone I get the dreaded exclamation point and the tracks have not copied and will not play. The problem has been intermittent but is getting reliably worse. As soon as a song fails to sync, the song in question receives an exclamation mark to its left in iTunes and an additional exclamation mark in a triangle appears in the left column, between my iPhone name and the rotating syncing icon.

To reiterate, this problem does not occur when I unselect the convert higher bitrate songs option, which means that it's to do with the conversion.

I also get the -54 error message intermittently.

Any ideas?