option to assign a letter to the HD is disabled...

Hey there,

It could be a couple of things:

1. What is the disk formatted as? NTFS, ExFat, etc?
2. On the left side you can right click the disk. From there you should see if it is in MBR or GPT mode. Some newer OS no longer support MBR or master boot record. Changing to GPT may fix it but requires a reformat of the drive.

This explains the difference between MBR and GPT:

https://www.tutorialspoint.com/differenc....