If you are running OS X Lion and Avast! for Mac Free in a Mac, as I am, you can instruct your avast! to avoid scanning a drive. Now, if you mean to stop avast! from scanning a drive, here is what I would do. (and, did.) 1st, to open your Avast! Preferences, click the left button of your mouse upon the “wrench” icon at upper right of the avast! window’s titlebar. At lower left corner of the avast! Preferences dialog box, click on the padlock icon and then authenticate with the password for the user profile you are logged into. The padlock will open, “unlocked.” Then, click on the tab labelled On-Demand Scans. On that tab, there is a checkbox labeled Scan also removable volumes. In my office, the Time Machine drive is an external, USB-connected, hard drive. In my opinion, all USB-connected drives can be described as “removable volumes.” If you will make sure that the checkbox for “removable volumes” is not selected with a checkmark in its box, then I believe that avast! ought to skip scanning that removable drive.
Be careful what you click upon, when the Padlock icon is unlocked. A click on the wrong item when unlocked can cause you a lot of unexpected trouble.
On your 2nd question, I must ask for information. What did you mean, by "MAC drive?"I can say that the T.M. volume should not be placed on the Mac’s internal, system hard drive. Generally, the T.M. should be on an external hard drive with capacity for two months or more. I would not want my Mac’s internal drive to have a setting that puts the Time Machine "on"my internal hard drive. It would slow down the system noticeably for quite a while whenever the T.M. would want to run a scheduled backup.
Did you want to stop the Time Machine from scanning a removable drive, in order to more quickly back up the System drive in your Mac? If you will open System Preferences (found on the Apple pull-down menu), you will see the Time Machine listed in the group designated the System group. A single, left click on any item in that list, will open its Preferences dialog box, so, click the Time Machine icon.
In the resulting Time Machine Preferences dialog, click the lock at lower left and it will cause OS X to request authentication, just as on avast! When the padlock opens, click the Options button. The pop-out that will appear then, contains the settings for designating any drives and also, any folders, to be skipped by the Time Machine when it runs for the purpose af making the backup.
I hope that this will be found helpful.