1. This "internal" hard drive would need to be formated initialy but if you'd redirect internal one to it PS3 will actually loose part of it's FW and all the files etc so I doubt you'd be even able to format it in a first place - it would most probably reboot and then try to reinstall missing part of FW from USB storage - the same way it happens when you replace / upgrade your internal hard drive. You can indeed try to format the hard drive internally first and then attach it via USB afterwards but I seriously doubt PS3 would even recognize it this way - respectivelly the external SATA - USB enclosure would be most probably not able to handle it at all.
2. Even if you'd manage that - every time you'd swap the "internal" hard drive on USB it will mess up with your system / seting etc.
And - if you'd jsut try to copy files accross - then the external hard drive must be format in FAT32 and we're back again on square one.
Not really sure how much consistent would be your PS3 after repeated messing with it like that ...
Two small "issues" with you idea though: