Because BIOS had to detect the drive, you had to have it in at boot. After that, you could take it out, put it in another system, power that one up, do stuff with the files, yank it, pop it back into your system, etc. So the only restriction was BIOS having to detect the drive.
<font color=blue>Only a place as big as the internet could be home to a hero as big as Crashman!</font color=blue>
<font color=red>Only a place as big as the internet could be home to an ego as large as Crashman's!</font color=red>