No letter assigned to USB mass storage device

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I'm having problems allocating a letter to external hard drive. Basically, I had an old computer that wasn't working properly but had some files on its hard disk that i wanted, so i removed it and bought a usb hard disk case to put it in. The hard drive is a Maxtor 3 2049H2 in case you were wondering and was assigned the letter C: on my old comp. When connecting the hard drive up to my laptop via USB it seemed to install fine and is recognised in the "safely remove hardware" options. It also appears in Computer Management Device Manager under the disk drives. However, it does not appear in Disk Management. I was wondering if anyone could help me get access to these files on my old hard drive? Thanks