I cannot see external hard drive on my computer, although it is shown in Disk Management. I read in other forums that I need to assign a drive letter, however when I right-click to assign all other options are grayed out except for Delete and Help, so I don't really know what to do at this point. Then I tried in the cmd window diskpart list volume, but the drive that is shown in Disk Management is not even listed in the command prompt. So I don't know what to do and how to use the drive.
The most likely explanation is that windows does not recognize the format of the external HDD. For example, it could be formatted as Linux's ext2, which Windows does not support.
If you do not have any data you need to extract from the drive, do the following:
- Open Disk Management
- Locate the removable disk
- Right-click on the Volume and select "Delete Volume"
- Right-click on the Unallocated space and select "New Simple Volume"
- Click Next, Next
- At this point you can name your volume and choose your file system.
For file system, I recommend NTFS unless you're planning on using the HDD on both Mac and Windows, then choose exFAT. I believe Fat32 is the most widely supported file system, but it limits your file size to 4GB.
If, on the other hand, you do have data that you want on the drive, you'll have to try booting it on a linux machine or using a linux-based live CD. Once in linux, you may be able to mount the drive, or you can at least use a utility such as GParted to figure out what file system your drive is formatted with.
It does show in disk management you say ?
What does it show exactly ? Can disk-management identify the partitions ?
Without further info I would say the disk was used on a Mac or Linux machine and is not formatted in any way that Windows will recognize.
(If you are using XP it will have the same issue if the disk was partitioned in a Windows 7 or 8 machine using a GPT partition table.)
There is another possibility too: Some USB external harddrives don't register with Windows as a harddrive, but as a memory-stick.
In that case Windows can only use them if there is exactly 1 partition on the drive. If there are 2 or more partitions it's not usable.
(This is a silly limitation in Windows. Other operating systems are just fine with this configuration.)