Buying external drives for the drive inside and using internally?

The two I can think of are that you void the warranty and have little control over the exact model that is contained within. Usually the warranties are one to two years on drives. At least since you are looking at an external made by a hard drive manufacturer you will know the brand being used though maybe not the exact model.

Other than that I have done this before on my Macbook. Since the brand I bought was known to carry WD Caviar Blues. Then you have the bonus of an external enclosure to pop your old drive in and use the internal for portable storage, scratch disk &c.

i was going to rip the drive out of a wd external i had lying around. just wanted a pvr extender (with an esata connection) - but the videos and tutorials i looked up on how to get into the wd case and then get the thing disconnected was way more complicated and physically difficult than i wanted to undertake.

so maybe check that out before you purchase.

More work than worth it

Unless you are an very very poor student, the 50 bucks you're going to save is not work it. Compatibility issues up the waazzooo! Unless you have all the equipment already set up to access the drive, get it into the right format etc....it's just not worth it!

Some WD portable aka laptop drives have a USB port soldered/connected directly to the drive making it practically impossible to connect to a Sata port in a Mac/PC. Search google for the exact model but I know WD has been doing this with all? portable drives for about 2 years, so a new drive will most likely have this caveat.