the raspberry pi IS a pc. adding it to yet another pc would just be totally redundant as it offers only a limited set of what the pc can already do but nothing more.
right now a typical scenario is:
PC -> Drive.
a usb scenario is typically:
PC -> USB converter -> Drive.
a high end motion controller scenario is:
PC -> Motion Controller -> Drive.
what you seem to be proposing is:
PC -> Raspberry Pi -> Drive Interfacre -> Drive.
it just seems like youre adding devices that dont actually add anything very useful.
what i would like is:
Raspberry Pi -> USB converter -> Drive.
the above yeilds something thats different than whats available now, and with several advantages, like cost and size and simplicity.