I've been using an expendable "test clone" on an external partition for years with no significant problems. I use the clone for all my tinkering, and then I just re-mirror it from my HD every once in a while using SuperDuper's "Smart Update" feature. This clone is of course completely separate from my "backup clones" which I never mess with. I would say that using a clone for testing is generally quite safe, but not 100% so - see this thread from earlier this year describing a rare but catastrophic issue experienced by some users that simultaneously destroyed the data on all their mounted volumes:
For extra safety you could routinely unmount your internal HD before doing any testing from the external volume, though I generally don't do this. What I do routinely do is turn off Time Machine and automatic Mobile Me syncing before creating the test clone so that these functions are inactive on the clone, and I also never try to sync an iOS device when booted from the clone.
Operationally the clone functions nearly normally, though what I do notice is that when an application that had been on the HD is run from the clone for the first time it is often again "quarantined" by the system as if it had just been downloaded, and I have to "approve" its use, even though I had cleared it from quarantine back on the HD before making the clone.
Thanks jsd2, very interesting, the links to the earlier posts too. I do in fact have three external drives only one of which is connected all the time for Time Machine. The other two are physically disconnected except when updating backups. Yes, got bootable clones as well as Time Machine.
My 'Name' fits just at the moment. I'm very close to the World Centre of Cricket and today is day 4 of a 5 day Test Match against Sri Lanka and the weather turned lousy and wet yesterday afternoon and remains like that so far. "Rain stopped play".