The blockchain is stored in the data directory, which can be configured with the datadir parameter. For example:
"C:\Program Files (x86)\Bitcoin\bitcoin-qt.exe" -datadir="E:\Data\Bitcoin"
Note that the wallet file is also stored in the data directory.
Symlinks may also be of use to you.
so that command will change the database to sit on the E: drive?
If you run Bitcoin with this command (as opposed to running it without this parameter), it will look for the database in the E:\Data\Bitcoin directory, and as necessary download it to that directory.
How you use it exactly depends on your exact setup, but assuming you have a Windows machine and a shortcut that points to "C:\Program Files (x86)\Bitcoin\bitcoin-qt.exe", and you want the database to reside in E:\Data\Bitcoin (or any other directory), you should:
1. Move your already existing database to E:\Data\Bitcoin.
2. Change the shortcut to point to "C:\Program Files (x86)\Bitcoin\bitcoin-qt.exe" -datadir="E:\Data\Bitcoin".