Sometimes, you can see the external hard drive in Disk Utility, but you can't use it, as you mentioned. This may be because the file system of the external hard drive is corrupted. You can use the following ways to check your disk.
Method 1: Verify you have set Finder to show External disks on the desktop. Click Finder in the Apple menu, choose Preference and make sure the External hard drive on the bottom is checked.
Method 2: Check whether your disk is mounted. Your external hard drive can't be accessed may be because it was not mounted. Therefore, you can try to use Disk Utility to ensure your disk is mounted. If the partition in Disk Utility is grayed out, you can manually mount it by selecting it and clicking the "Mount" button at the top toolbar.
Method 3: If you ensure your disk is mounted, there is nothing wrong with your disk. And the problem may be that you've currently booting from an APFS-formatted external hard drive and the partition you can't access from Finder is your system data volume. This volume will not show up in Finder or your desktop. So you need to make sure the type of your partition. To access this volume, you can run the following commands in Terminal.
- Note down the disk identifier of the partition you can't access (such as disk3s1).diskuitil list
- Get root access.sudo zsh
- Open the system volume.open/volume/disk_identifier
Method 4: If all the above solutions don't work, you need to format your disk. Before formatting your disk, you can use iBoysoft Data Recovery for Mac to take your data back.
If your partition is formatted with NTFS, and you don't want to reformat it, you can use iBoysoft NTFS for Mac to access your partition.
Also read:
How to Repair External Hard Drive Greyed out in Disk Utility?