You can boot it from a USB drive as long as it is partitioned properly and has the same version of your copy of OS X.
Mount an external hard drive on Mac Go to System Preferences > Disk Utility. Check that the external disk is listed in the left sidebar. Highlight your hard drive and select Mount. It should now appear under devices in the Finder.
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Connecting the Drive. Plug the hard drive into the Mac using the cable that came with it. Most hard drives connect via USB, so you'll just need to plug the USB cable into an open port on your Mac. You'll typically find at least one USB port along each side of the Mac.
Connect the external hard drive to your Mac. Open the Trash by clicking on its icon. Search for the file you want to recover. Right-click on the item and select the Put Back option to restore it to its original location on the external or internal drive.
Shut down your computer and then disconnect the external drive.How To Safely Disconnect an External Drive from your Computer
Use an external hard drive on a Macbook Air device Once you have an adapter, you can then connect the port of the latter to the port of your device. Macbook Air. You will then need to plug in the cord USB of your external hard drive into the corresponding port on the adapter of your device Macbook Air.
On your Mac, choose Apple menu > Log Out, then log in again. Try to eject the disc again. If you still can't eject the CD or DVD, choose Apple menu > Restart. While your computer restarts, press and hold the mouse or trackpad button until the disc is ejected.
If you can't eject the external hard drive, you can reboot or turn off your computer, and this will close all programs and processes to make sure no programs are using the files on the external hard drive. After your computer restarts or turns off, you can try to eject the external hard drive.
Absolutely, definitely do not remove spinning hard drives without ejecting them first. "They need to be warned that soon you'll take away their power—and therefore their ability to spin. "Even if a drive isn't USB-powered, it's still spinning, and it may be using files in the background," she says.
Go to the Start Menu, type in Disk Management and hit Enter. Locate the external hard drive that you want to eject. Right-click on your external hard drive and select 'Eject'.
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Any drive formatted with HFS+ will work just fine with a Mac that's running High Sierra or later. Neither Apple File System nor HFS+ works with Windows, however. If you plan to use your external drive with computers that run both operating systems, you should consider formatting your drive with the exFAT file system.
If the “This device is currently in use” error appears on your Windows 10 computer, try closing all files and programs on it. Then try to eject the removable device safely from the computer. Or you can refresh the desktop a few more times, and then try to eject the removable device.