Mac OS install drive not being recognized by computer I recently installed Mac OS 10.10 (Yosemite) onto an external USB drive. It has been working fine for the past few days. When I need to switch OS installs, I shut down the computer and then pull out the drive. However I think I just pulled out the drive a little too early, while it was still being written to, and now the drive is not being recognized at all by the computer. I get the following message. 



Is there any way I can still save the drive without losing all my work on it? Thanks.
You could plug it in, hit "Ignore", open "Disk Utility", and attempt to "Repair" the disk. Otherwise, try to initialize the disk and copy the folder contents. 

P.S. Just as a side note, I would recommend using an external hard drive(SSD or HDD) to run OS X, since it would operate a little faster.