150 Gig is an odd size. The whole "48-bit LBA Support" problem imposes a limit of 128 GB on a drive partitioned in an older system. But if yours really is 150 Gig, then somehow it got partitioned that way. Three possibilities for causes, and different fixes.
1. If it is simply Partitioned that way and you want to make it one huge volume containing the entire space of the 500 GB drive, you can do that ASSUMING that the empty unused space on it has no data you want. Windows has a tool to expand an existing Partition on a HDD by adding to it the Unallocated Space on that same drive, and the tool is in Disk Management I think. It will NOT do this on your boot drive. But yours appears to be a data drive only mounted in an external enclosure, so it should work.
2. If you have an odd external enclosure it is vaguely possible that its controller card can't handle a drive this size. But I have never heard of that showing up this way, so I doubt this is your problem.
3. I don't know if this last one applies to WD units; it does apply to Seagate, and MAYBE WD has a similar system. Seagate provides for free on their website a utility called Seatools for DOS. You can download a file and run it, and it will create a FLOPPY disk you can boot from to run a mini-DOS that runs their special Seatools software. One of its tools allows you to instruct the HDD to pretend to the world that its size is some number you specify smaller than the real size. From then on you will never suspect the real size of the HDD. This is useful in obscure situations (I've used it to make a 160 GB drive appear to be only 137 GB). The utility also has a related feature that tells the drive to revert to being its full original size when you want to un-do that limit. Now, IF WD has a similar feature and tools, maybe that was used sometime in the past to limit your drive. If not, forget this idea!