My Book Studio 2TB external hard drive- Apple Mac desk top os lion

I installed a mybook studio onto my desktop 12 months ago easy clear instructions  no problem. This is now full.

I purchased a new 2TB My book I have attempted to install it on my apple mac os lion but a : CANNOT DISMOUNT VOLUMES appears on the screen. I have turned everything off on the computer as per website trouble shooter but cannot install it, any ideas or should i take the hard drive back to the shop?

this happens if a file from the drive is open while formatting

make sure you close all files before formatting

you can restart the Mac and then format the drive