How to fix my external hard drive after connecting it to a pc? Possible USB 3.0 to USB 2.0 problem?
"With friends like that, who needs enemies?"

If you don't care about the stuff on the drive, you may still be able to erase and repartition it for use on your MacBook. In Disk Utility, select the drive in question, then click the Erase button. Choose Mac OS Extended (Journaled):



If all works as intended, then the drive will be usable again on your Mac.

If you want to be able to plug it into your friends PC to share files, you will need to erase it with a different format, this guide will help.

Essentially:


Launch Disk Utility
Connect the drive you wish to format for dual compatibility to the Mac
Click the drive name on the left side list in Disk Utility, and then click the “Erase” tab
Click the pulldown menu alongside “Format” and select “MS-DOS (FAT)”
Optionally, give the drive a name
Click the “Erase” button to format the drive for Mac &amp; Windows PC compatibility


NOTE: MS-DOS (FAT) format disallows files over 2 or 4 Gb in size. A better choice is ExFAT, especially if you're dealing with video files: