A greyed out external hard drive in Disk Utility indicates that the system can't mount the drive, so an unmounted disk won't show up either in Finder or on Mac desktop. To fix this issue, you can try to manually mount the Toshiba drive, update disk driver, repair drive errors, recover data and reformat the drive on Mac.
When external hard drive greyed out in Disk Utility, the drive is inaccessible, you can't read and write data to it. The causes behind greyed-out external hard drive in Disk Utility could be disk corruption, system errors, etc. When external hard drive not mounting on Mac, you can click go to Disk Utility, select the problematic drive and click Mount to manually mount the drive.
You should update macOS to the latest to make sure the disk driver is updated to support exterbal hard drives.
If the external hard drive remain greyed out, run Disk Utility's First Aid to check and repair disks. Here's how:
- Head to Disk Utility from Finder > Applications > Utilities.
- Select the greyed-out external drive from the left list.
- Click the First Aid button to start checking and repairing the disk.
- When the process completes, remount the drive again.
Unlucikly, the external hard drive is still unrecognizable on Mac, the last resort you can try is to format external hard drive on Mac, but it will erase all contents, therefore, you'd better recover lost/deleted data from the drive before you proceed to reformatting it, especially if there is no backup to restore the drive after reformatting. To make it, you need professional data recovery software such as iBoysoft Data Recovery for Mac. Download it and try it for free now!
If you have created backup for the greyed-put drive, you can directly erase it and then restore the drive from available backup. After reformatting, you should access the mounted drive on your Mac as usual.