Since the SDW 431S is a slim drive, this is not an issue.
Assuming, the internal drive is with Pata interface, there are two problem sources:
#1: Availability. Pata drives are rare these days.
#2: Master/Slave setting. Slim drives don’t have jumpers to adjust Master/Slave behaviour, that whole stuff is buried in the drive’s firmware. Especially if you buy some OEM drive, you might encounter problems here.
I personally would avoid that trouble and buy a new external drive. A “full sized” one, no slim drive. Price is about 50 USD nowadays.
Oops … its a slim drive indeed! :o
In that case things are even easier, as virtually every standard 12.7mm IDE drive will physically fit. The only change you might have to do is to swap bezels.
Regarding the master/slave setting … if you’re replacing it with another LiteOn drive that can be easily done using the EEPROM utility (in the ‘Slim’ tab) if necessary. Some NEC/Optiarc drives also have a master/slave setting utility available (make sure you check before buying one of those).