hi jeremy, can i check if the sentinel tool you used, seemed to run quite quickly (like about 15minutes) to scan each drive? or did it take several hours?
if it was a quicker test, it might be worth making a note of the current smart values, and then to run a full sector test as well, and then to compare smart values again, just to see.
one good tool you can also use (on new / empty drives) is a tool by western digital called data life guard which is free.
at first i was thinking that it could be just the fact that the fs is old, and maybe the drives you are using are special in some way, requiring a new firmware (which may not exist), though when you mentioned you already have 2 of them with the same identical model then it probably isnt that.
i would try some more tests if you can, just in case there actually are some sectors being reallocated too often for their own good, (which might help with the rma process too), but another theoretical test could also be the below, in 2 parts:
(please note, before you try this, if you do not have a backup of your data, you may want to take a backup, or at least of the most important data to play safe)
you could try powering all Off, and unplugging all cables from drobo
and remember the order of your diskpack for future, to swap 1 of the working models, with the drive that is in the slot of the drive you were trying to replace…and then to connect up and power up the drobo.
this way, your drobo should still be booting up with its diskpack drives inside,
but the toshiba drive (which was known to be working) would be in the slot where the replacement toshiba did not work and was rejected.
when the computer and dashboard boot up, and after say 10minutes of general usage, you could see if that (known to be working) toshiba you swapped bays with, gets rejected… if it does, then it could just be something about that model in that bay.
if the originally working toshiba, still works in its new bay, then there probably is something about the new toshiba that was sent to you, (especially if it is the same model and identical drive firmware etc) - also, dashboard may be able to show you more info and serial/firmware info about each drive, depending on version of dashboard.
the 2nd part of the test, (which will probably be best after letting your drobo run smoothlyfor a few days) would be to then try replacing (while powered on) the new, previously-rejected (and currently outside of the drobo) toshiba drive, by swapping it with the non-toshiba drive that you already swapped bays with in part 1)… if it accepts it here in this bay, then it could have just been a bay issue… but if it is still rejected, maybe there is something wrong with the drive.
btw for the part about wiping a drive, i think deleting the partition using a standalone connection with the drive to the computer, (without any drobo), to format it in windows, and then delete the partition, followed by using a tool like dariks boot and nuke has been able to wipe any flag, (but im not sure if some models of drobo firmware maintain a copy of the drives serial number)
if you do use dariks tool, it is best to run it on a computer that does not have any other drives or devices attached to it etc, apart from the drive you want to erase, just in case…
by the way, can i check how you have been trying to replace the drives so far?
is the drobo and dashboard on when you do this, or have you possibly been replacing while powered off, and then powering up and seeing it get rejected?