Some old BIOS are unable to recognize the full capacity of a new larger HDD and it can only address a part of the drive. You are probably using a 32-Bit host PC. Systems with an LBA length of 32 Bit only support storage capacities up to 2.2 TB. Toshiba recommend that you contact your PC manufacturer and get new BIOS that support your larger HDD. Please refer to the information on the document linked here (50KB).
If this is not the reason but the usable size is just a bit smaller than expected, this issue is probably due to different ways of calculating the size, either using the decimal or a binary system:
In a decimal system 1GB = 1,000MB = 1,000,000KB = 1,000,000,000Byte. But most operating systems calculate with factor 1024 instead of 1000: 1GB = 1024MB = 1024x1024KB = 1,073,741,824Byte. Because of this, all drives seem to be reduced about 7 percents smaller under the operating system than they really are.