Hi,
Thank you for posting your question in the Microsoft Community.
Based on the information, you are unable to open the external hard disk, as it wants to format it.
It would be helpful if you could reply to these questions to assist you further.
- What is the exact error message that you get?
- What is the File System of the External Hard Drive?
- Are you using the external hard drive for the first time on Windows 7 computer?
- Are you able to use other external drives without any issues?
- What happens when you connect the external devices to a different computer?
- Have you made any recent software or hardware changes on the system?
This issue may occur if the File system used in External hard drive is not supported In Windows 7.
Let’s try the following steps and check if it helps.
Step 1:
To check the Files System of External Hard Drive, follow the steps as below:
a. Click Start, click Computer.
b. Right click on the External Hard Drive and select properties.
Step 2:
Plug in the External Hard Drive in a different USB port and check if it is detected normally.
Step 3:
Copy/Backup all the files from External Hard Drive to another computer. Connect the External Hard Drive on Windows 7 computer and format it using NTFS File System. After that copy the data from to the External Hard Drive and check if it can be opened on Windows 7 computer normally.
a. Open Computer Management by clicking the Start button, clicking Control Panel, clicking System and Security, clicking Administrative Tools, and then double-clicking Computer Management.
If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
b. In the left pane, under Storage, click Disk Management.
c. Right-click the volume that you want to format, and then click Format.
d. To format the volume with the default settings, in the Format dialog box, click OK, and then click OK again.
Note: Converting a hard disk partition or volume from NTFS to FAT32 requires that you reformat the partition, which deletes any data on it. Be sure to back up any data you want to keep before you begin.
For further information, refer the articles:
Create and format a hard disk partition (Applicable for External Hard Drive as well)
Formatting disks and drives: frequently asked questions
Convert a hard disk or partition to NTFS format
Hope the information helps to fix the issue.
If you need further assistance, please reply and we will be happy to help you.