You most likely have installed the Grub bootloader to the internal disk, but the second stage of the bootloader resides on the external disk, which leads to your situation that you can boot only with the external disk attached.
To fix this attach the disk, boot into your Windows system and start a command shell as Administrator (open the start menu, type cmd
into the search bar and press Ctrl+Shift+Enter, then answer the UAC question with yes). Type in the command
Now the system should be able to boot without the external disk attached.
To make the disk usable for Windows again just use the Windows partition manager to remove the Linux partitions and create and format NTFS partitions.