The first caution is to make sure you have an owned and not leased DVR. If it is leased, you are not permitted to disassemble and repair it. Just call DirecTV and get a replacement.
If it is owned (not likely but possible), then about the only place you'll get the correct power supply is from another HR23-700.
I believe the screws on the cover are security torx. I don't know about the drive inside.