It propably has a file system that your computer doesn't recognize since you used it as a PVR. You need to open Disk Management (press windows key + x and select Disk Management), from there find the drive. Select the partition(s) on your external drive and delete them (this will also remove any data from the drive!). After it is done it should show the whole drive as empty space, now you right click on the empty space and select create partition. Then you need to format the partition (you can select NTFS if you only plan to use it with Windows devices or use FAT if you need to use with other systems aswell). After it finishes formatting your drive should show in My Computer.