How to initialize a bootable disk and keep it bootable?

I have a bootable hard disk from a dead Windows XP computer. I want to see what is in it and if I should get a replacement mother board for it. I hooked it up as a USB disk but it disk doesn’t show up in the file explorer. Right click “Computer” -> “Manage” -> “Disk Manage” asked me to initialize with the loss of data.

I saw some postings on how to do it without losing data. But I also want to keep the disk bootable in case I decide to get a new mother board.

I don’t understand the whole MBR thing, like what is in there, how to read and write to it etc. Some explanation would be very helpful and appreciated here.