Unfortunately, you cannot use generic PC equipment inside that 1U form-factor system.
Glance at the photo that is a part of the X2100 Components List in the SSH:
http://sunsolve.sun.com/handbook_pub/Systems/SunFireX2100/components.html#Remov ableMedia
Internal drives plug into their associated backplanes.
The drive backplane(s) connects across to the systemboard
for whatever necessary power and data connectors.
This design approach isn't suddenly new.
In Sun's product line, for example, it has been done at least since the Netra T105.
That T105 model system first appeared in mid-1999.
I'm sure there are many systems from other vendors that use the same design approach.