HD in caddy seems inactive and blocks application this is the premise:

Recently I removed from my mbp 15 (2012) the internal Superdrive and replaced it with a caddy + 1 Tb hard disk.
I already replaced the original HD with an SSD. Since I'm a programmer and I work intensively with my mac, I thought installing the caddy+HD was a good idea: this allows me to have intense write/read session over the HD rather than the ssd. Furthermore I moved all the general purpose files on the HD (music, documents and so on) by just copying it.

This said here is the strange thing. When I import music into iTunes from the HD it takes so long that the most times it start the color wheel and it blocks iTunes. Or while I'm working on a text file or source file it gets blocked while saving.

To me it looks like the hard disk goes to sleep from time to time (randomly????) and it blocks all the applications that are using it. The most obvious and tangible problem is given by the music player (that's why I spotted this problem). Sometime for example it is playing the music and by skipping from one song another it blocks for a while (keep in mind that I don't import things in the iTunes library cause this means a raw copy of the same things.. ).

Any clue is appreciated in spotting any problem or misconfiguration or whatever..
In system preferences -> energy saving you can disable putting HDDs to sleep. If that doesn't help my first guess would be a faulty cable or caddy.