welcome to PCHF crttop
I have merged your 2 posts as they both asked essentially the same question... this not only delays your answers-- but ties up resources that others could be using. Please avoid double-posting.
As long as your external HD is in a removable case, and it is a standard drive type (IDE ? SATA ?) you can use it any way you wish. Master/Slave/Cable-select-- your motherboard must have an open connector for whatever drive-type it is, also. SATA drives won't work if your
MOBO does not have SATA connectors. Almost ALL Modern MOBOs have IDE connectors.
I think I've covered everything....