Maybe I can help you, clara456. That's a mighty good pair of questions...... Time Machine, when set up, will NOT erase everything in your external hard drive, unless you tell it which drive you want to use as a Backup, except for the first time....
and....on to your 2nd question...... Yes, you will still be able to use that hard drive on windows laptops as well as on Mac, depending somewhat on its size. I'm presuming the external hard drive in question has sufficient size (capacity) for your needs. What you would have to do is connect it to your Mac via USB or whichever connector it has... I'm guessing it's USB, and then tranfer any important files from the external hard drive to your Mac somewhere. Maybe you want to make a new folder, name it and then transfer the stuff there, I don't know.
Having transferred any important files from the external hard drive to your Mac, you are now ready to prepare it for Time Machine use, and for use with windows laptops. You can do this in Disk Utility by creating 2 partitions. Click on the + plus sign to do so. You can resize each partition before you make it, and set its format. On the Mac side, it should be MacOS Extended (case sensitive) for the Time Machine backup, and for the Windows Side, ExFat. You might want to give each Partition a name so you can keep track of which is which. You should let everyone else in your house who uses Windows know, that Yes, they can use your external hard drive to transfer stuff to each other, but that they should put it into the appropriate partition dedicated to Windows (the ExFat one), and of course, you'll be following the protocol for external hard drives, that is, after you have finished using it, and transferrring files, and it's done, that you eject it, wait until it has been safely ejected-it'll disappear off of the Desktop, and then you can unplug it. Of course, under Finder, then Preferences, you can choose to show certain items on the Desktop, Hard disks, external Disks, CD's DVD's and Connected Servers, but I'm sure, being a smart cookie, you already knew and did that already.
hope this helps you
john b
