How to copy a file into Micro SD via USB in Android?

You don't mention which Android phone you've got, depending on which phone (and how much your manufacturer's customised) you've got, it may be slightly different from OtherMichael's instructions.

With stock Android (as on the Nexus One, HTC Magic, G1, etc), when you plug your phone into the PC's USB you should see a USB icon appear in the notification area. Pull the notification area down and you should see "USB connected: Select to copy files to/from your computer." Press this and you get the option to "Mount". Pressing this connects your phone's SD to the PC.

Other manufacturers customize things slightly with their custom UIs, for instance if you have a phone with Samsung's TouchWiz UI on it, you get a menu up as soon as you plug into USB, with options to select USB Mode, on the you'd pick Mass Storage, and then do as above. I think HTC's Sense UI also customizes things slightly, but I can't remember exactly how that works.

Settings >> Connect to PC >> USB Connection Type

should be set to "Disk drive" (you can set this as a default connection type, but I prefer to do it manually when I need it).

Also, you can set it via the notifcations bar by clicking on the USB connection type as it appears; a selection box of the various options should appear.

Once mounted, you can open on your PC, and drag-n-drop, or what-have-you.

NOTE (from the Incredible manual, p29):

Disk drive

This mode is available only when you have installed a microSD card on your phone. Select this mode when you want to use your storage card as a USB thumb drive, and copy files between your phone’s storage card and your computer.

When Disk Drive mode is enabled while your phone is connected to the computer, you will not be able to run applications that use the storage card, such as Music or Photos. For more information, see “Copying files to and from the storage card” in this chapter.