Broken capacitor off of drive power board on ReadyNas Pro Pioneer

Based on the visible traces on the board, it's for transient/surge protection on the 3.3V SATA power line.  Since I know of no SATA device that uses the 3.3V, I believe everything will work just fine without it.  The early 4-bay units originally had those installed, but later ones don't (there is just an empty location for them).  I'm surprised they didn't do a cost cutting for the 6-bay units and remove them as well, but even my Ultra6Plus has them. 

The middle one (as oriented in your picture) is for 5V and the left one for 12V, so you got lucky.  All of them are mainly needed for hot swapping of the drives, anyway.  Standard SATA wired connections don't have them.  You will also note that one pin for each voltage has an inline resistor.  Those go to the drive connector pins that are a little longer, so engage first, and are for "pre-charging" during a hot swap.  Again, a wired connection doesn't have them (which is why you should never hot swap drives without an appropriate hot swap bay).

If you do want to replace it, please talk to an electronics technician, not an electrician.  The former deals with circuit boards, and the later deals with building wiring.  The numbering on the capacitor is nothing I am familiar with or can Google, so unless there is additional writing on the side that gives the value or a Netgear employee with access to the schematic will tell you the value, he will likely need to measure the capacitance on one of the others and guess at the voltage rating based on it being for 3.3V and the physical size.  The value can likely be read accurately in situ rather than having to remove another for measurement.  On the 4-bay units, the numbering is more typical and they are 100uF 25V electrolytics, which is a typical value for that use.  Since the 12V cap in your picture has a different part number from the other two, the 5V and 3.3V may be rated at 15V with only the 12V rated for 25V as a cost savings (you typically use a cap rated at least 2x the actual voltage), but 25V for all sould work fine.  Or, the capacitance could actually be different for some reason  You can buy 10 of the 100uF 25V for $1.65 on eBay.