OS sees disk as smaller than it is; did Secure Erase succeed?

Question

I had to return a defective 4TB (2x2TB) RAID drive to the manufacturer.
It had been hooked up to a 27" iMac i7 with 10.6.x, but I attached it to my PowerMac G5 in order to do a Secure Erase (we were having brownouts in the area and I was less worried about leaving my old G5 on for the day+ needed).
I did the 1-pass secure erase. However, I noticed that the G5 (dual 2GHz running 10.5.8) reported the drive as being 3.6 TB; the iMac reported it as being the full 4TB.
So,
1) why the apparent difference in size?
2) more importantly, did all of the data (including Social Security numbers, credit card info, online banking/investing data, etc.) securely erase, or would this mean that some area of the drive might have been missed?

Answer 1

1000 * 1000 * 1000 * 1000 * 1000  = 1000000000000000
1024 * 1024 * 1024 * 1024  * 1024 = 1125899906842624
These terms are usually used in the world             of computing to describe disk space, or data storage space,             and system memory. For instance, just a few years ago we             were describing hard drive space using the term Megabytes.             Today, Gigabytes is the most common term being used to describe             the size of a hard drive. In the not so distant future,             Terabyte will be a common term. But what are they? This             is where it gets quite confusing because there are at least             three accepted definitions of each term.         
According to the IBM Dictionary of computing, when used             to describe disk storage capacity, a megabyte is 1,000,000             bytes in decimal notation. But when the term megabyte is used for                                     real and virtual storage, and channel volume, 2 to the 20th power or 1,048,576 bytes is the appropriate notation.                        According to the Microsoft Press             Computer Dictionary, a megabyte means either 1,000,000 bytes             or 1,048,576 bytes. According to Eric S. Raymond in The             New Hacker's Dictionary, a megabyte is always 1,048,576             bytes on the argument that bytes should naturally be computed             in powers of two. So which definition do most people conform             to?
When referring to a megabyte for disk storage,             the hard drive manufacturers use the standard that a megabyte             is 1,000,000 bytes. This means that when you buy an 80 Gigabyte             Hard drive you will get a total of 80,000,000,000 bytes             of available storage. This is where it gets confusing because             Windows uses the 1,048,576 byte rule so when you look at             the Windows drive properties an 80 Gigabyte drive will report             a capacity of 74.56 Gigabytes and a 250 Gigabyte drive will only yield 232 Gigabytes of  available storage space. Anybody confused yet? With             three accepted definitions, there will always be some confusion             so I will try to simplify the definitions a little.
The 1000 can be replaced with 1024 and still             be correct using the other acceptable standards. Both of these standards are correct depending on what type of storage you are referring.
http://www.whatsabyte.com/